Density processing method

ABSTRACT

The present invention is directed to a density processing method in which input gray level-output gray level data are previously generated and stored a first storing device, the input gray level-output gray level data are transferred to a second storing device, and an address in the second storing device is designated on the basis of input gray level data inputted so that output gray level data stored in the specified address in the second storing device is outputted. In the present invention, access addresses to the second storing device are converted depending on the designated copy density, so that input gray level-output gray level characteristics are converted into characteristics corresponding to the designated copy density.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a density processing method in an imageforming apparatus such as a copying machine.

2. Description of the Prior Art

In a digital color copying machine, a document is first irradiated by anexposure lamp in a scanner section, and its reflected light is detectedby a CCD (Charge Coupled Device) and sequentially converted into anelectric signal. In this case, an image of the document is separated foreach color and for each pixel by the CCD into an electric signalcorresponding to the density of each pixel. This electric signal is sentto an image processing section.

In the image processing section, an output of the CCD is digitallyconverted and then, the variations of the CCD, the exposure lamp and thelike are corrected for each signal having colors (B (blue), G (green)and R (red)) (BGR signal) in a shading correcting section. The BGRsignal is converted into a toner density signal (a YMC (yellow, magentaand cyan) signal) in a BGR-YMC converting section. In addition, a BK(black) signal is produced from the YMC signal in a BK producingsection.

Thereafter, in the YMC signal and the BK signal, the density levels ofthe respective colors are corrected in accordance with thecharacteristics of a filter and toner in a color correcting section.Furthermore, in a color converting section, processing, such as colorconversion of a designated color, trimming or masking, is performed.Thereafter, in a density processing section, the level of a digitaldensity signal sent is converted depending on a developing color, thecopy density designated in an operating section, the type of documentimage designated in the operating section, and the like. Thereafter, thedigital density signal is sent to a printing section through a variablemagnification and movement processing section for performing variablemagnification and movement processing in the main scanning direction ofan image, to accomplish recording on paper.

Meanwhile, in the digital copying machine or the digital color copyingmachine, density processing using a dither method has been generallyperformed in the density processing section so as to obtain a gray scaleimage. A digital color copying machine already developed by theapplicant of the present application is so adapted that datarepresenting a gray level of a recording pixel relative to a gray levelof a reading pixel (hereinafter referred to as input gray level-outputgray level data) have been previously generated using a dither matrixhaving 2×2 pixels as one block and stored in a memory device, to finddata representing a gray level of a recording pixel (hereinafterreferred to as output gray level data) corresponding to datarepresenting a gray level of a reading pixel (hereinafter referred to asinput gray level data) inputted on the basis of the input graylevel-output gray level data. There are 256 gray levels of the readingpixel, and there are 64 gray levels of the recording pixel. A densityprocessing circuit in this copying machine is shown in FIG. 1.

The density processing circuit comprises a CPU 10, a data ROM 11, atable memory 13, and an address generating circuit 12. A plurality oftypes of input gray level-output gray level data previously generatedare stored in the data ROM 11 depending on a developing color (M, C, Yor BK), the copy density designated in an operating section, and thetype of document image designated in the operating section.

The CPU 10 transfers one type of input gray level-output gray level datacorresponding to the developing color, the copy density designated inthe operating section, and the type of document image designated in theoperating section out of the plurality of types of input graylevel-output gray level data from the data ROM 11 to the table memory13. Image data representing a gray level of reading pixel (input graylevel data) and a signal representing the position of the reading pixel(a pixel position signal) comprising a line signal HSYNC and a dotsignal CLK (not shown) are sent to the address generating circuit 12.The address generating circuit 12 outputs a signal for specifying anaddress storing output gray level data corresponding to the input graylevel data and the pixel position signal sent (a pixel in the dithermatrix which corresponds to the reading pixel) out of addresses in thetable memory 13. Consequently, the output gray level data stored at thespecified address is outputted from the table memory 13.

In this copying machine, the number of gray levels of the reading pixelis 256, and the number of pixels constituting one block in the dithermatrix is 4. Accordingly, the number of one type of input graylevel-output gray level data is 256×4=1024. Such input gray level-outputgray level data differ for each developing color (M, C, Y or BK), foreach copy density designated in the operating section, and for each typeof document image designated in the operating section. It is assumedthat there are four types of developing colors, there are 15 levels ofthe copy density designated in the operating section, and there arethree types of document images designated in the operating section, thatis, a character, a photograph and a character-photograph mixture. Inthis case, if the data ROM 11 is a 8-bit memory, 1024×4×15×3=184320bytes is required as the capacity of the data ROM 11. In such a copyingmachine, therefore, a large-capacity memory device is required to storethe input gray level-output gray level data.

Therefore, the applicant of the present application has proposed amethod of generating input gray level-output gray level data for eachdeveloping color (M, C, Y or BK) and for each type of document imagedesignated in the operating section and making the adjustment of anoutput gray level based on the copy density designated in the operatingsection on the basis of the input gray level-output gray level datagenerated so as to reduce the capacity of the data ROM 11. Specifically,it is a method of finding an output gray level relative to an input graylevel on the basis of input gray level-output gray level datacorresponding to a developing color and the type of document imagedesignated in the operating section and then, increasing or decreasingthe output gray level found by a gray level corresponding to the copydensity designated in the operating section, to obtain an output graylevel. That is, it is a method of merely adding or subtracting an offsetvalue to or from the output gray level found by the input graylevel-output gray level data.

FIG. 19 shows input gray level-output gray level data corresponding to adeveloping color and the type of document image designated in theoperating section, and FIG. 20 shows input gray level-output gray leveldata obtained by so correcting the input gray level-output gray leveldata shown in FIG. 19 that output gray level data is larger by apredetermined gray level by the above described method.

It is preferable that the relationship between input gray level data andthe density at which printing is actually done is linear. However, therelationship between output gray level data and the density at whichprinting is actually done is not linear due to characteristics which areinherent to the copying machine. Accordingly, if the relationshipbetween the input gray level data and the output gray level data is madelinear, the relationship between the input gray level data and thedensity at which printing is actually done is not linear. Therefore,input gray level-output gray level data is so generated that therelationship between input gray level data and the density at whichprinting is actually done is linear in consideration of the relationshipbetween output gray level data and the density at which printing isactually done which is inherent to the copying machine. Therefore, thecharacteristics of the input gray level-output gray level data generatedbecome non-linear characteristics and characteristics which are inherentto output gray levels.

In a method of merely adding or subtracting an offset value to or froman output gray level found by input gray level-output gray level data,if input gray level-output gray level characteristics obtained by themethod shown in FIG. 20 are compared with the original input graylevel-output gray level characteristics shown in FIG. 19, the value ofan output gray level at a point on the characteristic curve is changedfrom the value of an output gray level at a corresponding point on theoriginal characteristic curve. Accordingly, the relationship between theoutput gray level data and the density at which printing is actuallydone in the original input gray level-output gray level characteristicscannot be maintained. Therefore, the relationship between the input graylevel data and the density at which printing is actually done is notlinear. Further, in the method of merely adding or subtracting an offsetvalue to or from an output gray level found by input gray level-outputgray level data, the range of output gray levels is extremely decreased.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a density processingmethod in which the capacity of a memory device for storing datarepresenting an output gray level relative to an input gray level(hereinafter referred to as input gray level-output gray level data) canbe reduced, and the range of output gray levels is not extremelydecreased and the relationship between input gray level data and thedensity at which printing is actually done can be made linear, ascompared with a method of merely adding or subtracting an offset valueto or from an output gray level found by input gray level-output graylevel data to obtain an output gray level corresponding to the copydensity.

In a density processing method in which input gray level-output graylevel data are previously generated and stored in first storing means,the input gray level-output gray level data are transferred to secondstoring means, and an address in the second storing means is specifiedon the basis of input gray level data inputted so that output gray leveldata stored in the specified address in the second storing means isoutputted, a first density processing method according to the presentinvention is characterized in that access addresses to the secondstoring means are converted depending on the designated copy density toconvert input gray level-output gray level characteristics intocharacteristics corresponding to the designated copy density.

The access addresses to the above described second storing means aretransfer destination addresses from the first storing means to thesecond storing means corresponding to the input gray level-output graylevel data or specified addresses in the second storing meanscorresponding to the input gray level data inputted.

The access addresses to the second storing means may be converted byadding or subtracting a value corresponding to the designated copydensity, to convert the input gray level-output gray levelcharacteristics into characteristics corresponding to the designatedcopy density.

Furthermore, the access addresses to the second storing means may be soconverted, if the designated copy density is higher than the referencecopy density, that the rate of increase of an output gray level relativeto an input gray level is higher in a lower density portion and that thehigher the designated copy density is, the higher the rate of increasethereof is, to convert the input gray level-output gray levelcharacteristics into characteristics corresponding to the designatedcopy density.

Additionally, the access addresses to the second storing means may be soconverted, if the designated copy density is lower than the referencecopy density, that the rate of decrease of an output gray level relativeto an input gray level is higher in a higher density portion and thatthe lower the designated copy density is, the higher the rate ofdecrease thereof is, to convert the input gray level-output gray levelcharacteristics into characteristics corresponding to the designatedcopy density.

Furthermore, the access addresses to the second storing means may be soconverted, if the designated copy density is higher than the referencecopy density, that the rate of increase of an output gray level relativeto an input gray level is higher in a lower density portion and that thehigher the designated copy density is, the higher the rate of increasethereof is, and may be so converted, if the designated copy density islower than the reference copy density, that the rate of decrease of anoutput gray level relative to an input gray level is higher in a higherdensity portion and that the lower the designated copy density is, thehigher the rate of decrease thereof is, to convert the input graylevel-output gray level characteristics into characteristicscorresponding to the designated copy density.

In a density processing method in which input gray level-output graylevel data are previously generated and stored in first storing means,the input gray level-output gray level data are transferred to secondstoring means, and an address in the second storing means is specifiedon the basis of input gray level data inputted so that output gray leveldata stored in the specified address in the second storing means isoutputted, a second density processing method according to the presentinvention is characterized by comprising the steps of convertingtransfer destination addresses to the second storing means correspondingto the input gray level-output gray level data depending on thedesignated copy density, finding, when blank transfer destinationaddresses which do not exist in transfer destination addresses obtainedby the conversion exist in portions assigned small addresses of storageareas holding the input gray level-output gray level data in the secondstoring means, output gray level data corresponding to the blanktransfer destination addresses on the basis of output gray level datacorresponding to the minimum value of the transfer destination addressesobtained by the conversion, and generating new output gray level datacorresponding to all the storage areas holding the input graylevel-output gray level data in the second storing means from theforegoing steps to transfer the same to the second storing means.

In a density processing method in which input gray level-output graylevel data are previously generated and stored in first storing means,the input gray level-output gray level data are transferred to secondstoring means, and an address in the second storing means is specifiedon the basis of input gray level data inputted so that output gray leveldata stored in the specified address in the second storing means isoutputted, a third density processing method according to the presentinvention is characterized by comprising the steps of convertingtransfer destination addresses to the second storing means correspondingto the input gray level-output gray level data depending on thedesignated copy density, finding, when blank transfer destinationaddresses which do not exist in transfer destination addresses obtainedby the conversion exist in portions assigned large addresses in storageareas holding the input gray level-output gray level data in the secondstoring means, output gray level data corresponding to the blanktransfer destination addresses on the basis of output gray level datacorresponding to the maximum value of the transfer destination addressesobtained by the conversion, and generating new output gray level datacorresponding to all the storage areas holding the input graylevel-output gray level data in the second storing means from theforegoing steps to transfer the same to the second storing means.

In a density processing method in which input gray level-output graylevel data are previously generated and stored in first storing means,the input gray level-output gray level data are transferred to secondstoring means, and an address in the second storing means is specifiedon the basis of input gray level data inputted so that output gray leveldata stored in the specified address in the second storing means isoutputted, a fourth density processing method according to the presentinvention is characterized by comprising the steps of convertingtransfer destination addresses to the second storing means correspondingto the input gray level-output gray level data depending on thedesignated copy density, finding, when blank transfer destinationaddresses which do not exist in transfer destination addresses obtainedby the conversion exist in portions assigned small addresses in storageareas holding the input gray level-output gray level data in the secondstoring means, output gray level data corresponding to the blanktransfer destination addresses on the basis of output gray level datacorresponding to the minimum value of the transfer destination addressesobtained by the conversion, finding, when blank transfer destinationaddresses which do not exist in the transfer destination addressesobtained by the conversion exist in portions assigned large addresses inthe storage areas holding the input gray level-output gray level data inthe second storing means, output gray level data corresponding to theblank transfer destination addresses on the basis of output gray leveldata corresponding to the maximum value of the transfer destinationaddresses obtained by the conversion, and generating new output graylevel data corresponding to all the storage areas holding the input graylevel-output gray level data in the second storing means from theforegoing steps to transfer the same to the second storing means.

In a density processing method in which input gray level-output graylevel data are previously generated using a dither matrix and stored infirst storing means, the input gray level-output gray level data aretransferred to second storing means, and an address in the secondstoring means is specified on the basis of input gray level datainputted so that output gray level data stored in the specified addressin the second storing means is outputted, a fifth density processingaccording to the present invention is characterized by comprising thesteps of converting transfer destination addresses to the second storingmeans corresponding to the input gray level-output gray level datadepending on the designated copy density, finding, when blank transferdestination addresses which do not exist in transfer destinationaddresses obtained by the conversion exist in portions assigned smalladdresses in storage areas holding the input gray level-output graylevel data in the second storing means, patterns of output gray leveldata corresponding to the blank transfer destination addresses on thebasis of the original input gray level-output gray level data to findthe output gray level data corresponding to the blank transferdestination addresses on the basis of the patterns found and output graylevel data corresponding to the minimum value of the transferdestination addresses obtained by the conversion, and generating newoutput gray level data corresponding to all the storage areas holdingthe input gray level-output gray level data in the second storing meansfrom the foregoing steps to transfer the same to the second storingmeans.

In a density processing method in which input gray level-output graylevel data are previously generated using a dither matrix and stored infirst storing means, the input gray level-output gray level data aretransferred to second storing means, and an address in the secondstoring means is specified on the basis of input gray level datainputted so that output gray level data stored in the specified addressin the second storing means is outputted, a sixth density processingmethod according to the present invention is characterized by comprisingthe steps of converting transfer destination addresses to the secondstoring means corresponding to the input gray level-output gray leveldata depending on the designated copy density, finding, when blanktransfer destination addresses which do not exist in transferdestination addresses obtained by the conversion exist in portionsassigned large addresses in storage areas holding the input graylevel-output gray level data in the second storing means, patterns ofoutput gray level data corresponding to the blank transfer destinationaddresses on the basis of the original input gray level-output graylevel data to find the output gray level data corresponding to the blanktransfer destination addresses on the basis of the patterns found andoutput gray level data corresponding to the maximum value of thetransfer destination addresses obtained by the conversion, andgenerating new output gray level data corresponding to all the storageareas holding the input gray level-output gray level data in the secondstoring means from the foregoing steps to transfer the same to thesecond storing means.

In a density processing method in which input gray level-output graylevel data are previously generated using a dither matrix and stored infirst storing means, the input gray level-output gray level data aretransferred to second storing means, and an address in the secondstoring means is specified on the basis of input gray level datainputted so that output gray level data stored in the specified addressin the second storing means is outputted, a seventh density processingmethod according to the present invention is characterized by comprisingthe steps of converting transfer destination addresses to the secondstoring means corresponding to the input gray level-output gray leveldata depending on the designated copy density, finding, when blanktransfer destination addresses which do not exist in transferdestination addresses obtained by the conversion exist in portionsassigned small addresses in storage areas holding the input graylevel-output gray level data in the second storing means, patterns ofoutput gray level data corresponding to the blank transfer destinationaddresses on the basis of the original input gray level-output graylevel data to find the output gray level data corresponding to the blanktransfer destination addresses on the basis of the patterns found andoutput gray level data corresponding to the minimum value of thetransfer destination addresses obtained by the conversion, finding, whenblank transfer destination addresses which do not exist in the transferdestination addresses obtained by the conversion exist in portionsassigned large addresses in the storage areas holding the input graylevel-output gray level data in the second storing means, patterns ofoutput gray level data corresponding to the blank transfer destinationaddresses on the basis of the original input gray level-output graylevel data to find the output gray level data corresponding to the blanktransfer destination addresses on the basis of the patterns found andoutput gray level data corresponding to the maximum value of thetransfer destination addresses obtained by the conversion, andgenerating new output gray level data corresponding to all the storageareas holding the input gray level-output gray level data in the secondstoring means from the foregoing steps to transfer the same to thesecond storing means.

In the first to seventh density processing methods according to thepresent invention, it is possible to reduce the capacity of the memorydevice for storing the input gray level-output gray level data.Moreover, the range of output gray levels is not extremely decreased andthe relationship between the input gray level data and the density atwhich printing is actually done can be made linear, as compared withthat in a method of merely adding or subtracting an offset value to orfrom an output gray level found by input gray level-output gray leveldata to obtain an output gray level corresponding to the copy density.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electrical block diagram showing a density processingcircuit;

FIG. 2 is a schematic diagram showing four pixels in a dither matrix;

FIG. 3 is a schematic diagram showing the contents of a data ROM 11;

FIG. 4 is a schematic diagram showing the interior of a table memory 13;

FIG. 5 is a graph for explaining a first method;

FIG. 6 is a graph showing input gray level-output gray levelcharacteristics in a case where the first method is used;

FIG. 7 is a graph for explaining a second method;

FIG. 8 is a graph showing input gray level-output gray levelcharacteristics in a case where the second method is used;

FIG. 9 is a graph for explaining a third method;

FIG. 10 is a graph showing input gray level-output gray levelcharacteristics in a case where the third method is used;

FIG. 11 is a graph showing input gray level-output gray levelcharacteristics for explaining a fifth method and a sixth method in acase where the designated copy density is lower than the reference copydensity;

FIG. 12 is a graph showing input gray level-output gray levelcharacteristics obtained by the fifth and sixth methods in a case wherethe designated copy density is lower than the reference copy density;

FIG. 13 is a graph showing input gray level-output gray levelcharacteristics for explaining the fifth and sixth methods in a casewhere the designated copy density is higher than the reference copydensity;

FIG. 14 is a graph showing input gray level-output gray levelcharacteristics obtained by the fifth and sixth methods in a case wherethe designated copy density is higher than the reference copy density;

FIG. 15 is a graph showing input gray level-output gray levelcharacteristics for explaining a seventh method;

FIG. 16 is a graph showing input gray level-output gray levelcharacteristics obtained in a case where the seventh method is used;

FIG. 17 is a graph showing input gray level-output gray levelcharacteristics for explaining an eighth method;

FIG. 18 is a graph showing input gray level-output gray levelcharacteristics obtained when the eighth method is used;

FIG. 19 is a graph showing one example of the original input graylevel-output gray level characteristics;

FIG. 20 is a graph showing input gray level-output gray levelcharacteristics obtained by shifting the original input graylevel-output gray level characteristics shown in FIG. 19 upward.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Description is now made of an embodiment in a case where the presentinvention is applied to a digital color copying machine with referenceto the drawings.

FIG. 1 shows a density processing circuit in a digital color copyingmachine.

The density processing circuit comprises a CPU 10, a data ROM 11, atable memory 13, and an address generating circuit 12. The CPU 10comprises a RAM 14 storing necessary data. A plurality of types of datarepresenting a gray level of a recording pixel relative to a gray levelof a reading pixel (hereinafter referred to as input gray level-outputgray level data) are stored in the data ROM 11 depending on a developingcolor (M, C, Y or BK) and the type of document image designated in anoperating section. The document images are of three types, that is, acharacter, a photograph, and a character-photograph mixture. The inputgray level-output gray level data are previously generated using adither matrix having as one block 2×2 pixels G₀, G₁, G₂ and G₃ shown inFIG. 2. There are 256 gray levels of the reading pixel, and there are 64gray levels of the recording pixel.

Input gray level-output gray level data corresponding to a developingcolor and the type of document image out of the plurality of types ofinput gray level-output gray level data stored in the data ROM 11 aretransferred to the table memory 13 by the CPU 10. Data representing agray level of a reading pixel (hereinafter referred to as input graylevel data) and a signal representing the position of the reading pixel(hereinafter referred to as a pixel position signal) comprising a linesignal HSYNC and a dot signal CLK (not shown) are sent to the addressgenerating circuit 12.

The address generating circuit 12 outputs a 10-bit signal for specifyingan address (hereinafter referred to as an address specifying signal)expressed by binary numbers corresponding to the input gray level dataand the pixel position signal (a pixel in the dither matrix whichcorresponds to the reading pixel). The lower eight bits of the addressspecifying signal correspond to the input gray levels 0 to 255, and theupper two bits thereof correspond to each of the pixels G₀, G₁, G₂ andG₃ in the dither matrix. Values expressed by the upper two bits of theaddress specifying signal which correspond to each of the pixels G₀, G₁,G₂ and G₃ are "00", "01", "10" and "11". When the address specifyingsignal is outputted from the address generating circuit 12, data storedin a specified address in the table memory 13 is outputted as datarepresenting an output gray level (hereinafter referred to as outputgray level data) from the table memory 13.

Table 1 shows one example of the input gray level-output gray leveldata. In Table 1, base addresses Oadr 0 to 255 denote specifiedaddresses used as the basis corresponding to input gray level data 0 to255, and the base addresses Oadr 0 to 255 have a one-to-onecorrespondence with the input gray level data 0 to 255. In addition, atotal gray level indicates the total of output gray levels correspondingto the four pixels in the dither matrix relative to an input gray level.

                  TABLE 1                                                         ______________________________________                                                   BASE       OUTPUT GRAY  TOTAL                                      INPUT GRAY ADDRESS    LEVEL DATA   GRAY                                       LEVEL DATA (Oadr)     G.sub.0                                                                             G.sub.1                                                                            G.sub.2                                                                           G.sub.3                                                                           LEVEL                                ______________________________________                                        0          0          0     0    0   0   0                                    1          1          1     0    0   0   1                                    2          2          2     0    0   0   2                                    .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    64         64         64    0    0   0   64                                   65         65         64    1    0   0   65                                   .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    129        129        64    64   0   1   129                                  .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    255        255        64    64   63  64  255                                  ______________________________________                                    

FIG. 3 shows the contents of the data ROM 11.

In this copying machine, input gray level-output gray level data arepreviously generated for four types of colors (M, C, K, and BK) and forthree types of document images, that is, a character, a photograph and acharacter-photograph mixture designated in the operating section.Specifically, 12 types of input gray level-output data are respectivelystored in areas RE₀ to RE₁₁ in the data ROM 11. In this example, thedata ROM 11 is a 8-bit memory. Accordingly, the number of bytescomposing each of the areas RE₀ to RE₁₁ is (the number of input graylevels)×(the number of pixels in the dither matrix), that is, 1024.Consequently, the number of bytes composing all the 12 areas is1024×12=12288.

FIG. 4 shows the interior of the table memory 13.

The table memory 13 is a 8-bit memory, and comprises an area TE₀(addresses 0 to 255) for storing input gray level-output gray level data(256 gray levels) with respect to the pixel G₀ in the dither matrix, anarea TE₁ (addresses 256 to 511) for storing input gray level-output graylevel data with respect to the pixel G₁, an area TE₂ (addresses 512 to767) for storing input gray level-output gray level data with respect tothe pixel G₂, and an area TE₃ (addresses 768 to 1023) for storing inputgray level-output gray level data with respect to the pixel G₃.

The density processing circuit in the copying machine outputs outputgray level data corresponding to the copy density designated in theoperating section on the basis of the 12 types of input graylevel-output gray level data stored in the data ROM 11. There are ninemethods thereof. Description is now made of the respective methods.

(I) First Method

As shown in FIG. 6, the first method is one for shifting, in a casewhere input gray levels are used to enter the axis of abscissa, outputgray levels are used to enter the axis of ordinate, and the originalinput gray level-output gray level characteristics (where output graylevels denote total output gray levels) are indicated by a graph line a,the graph line a to the left or right depending on the designated copydensity, to convert the original input gray level-output gray levelcharacteristics into characteristics corresponding to the designatedcopy density (a graph line b or c). There are two methods used forcarrying out the first method.

(I-1) First method (1)

The first method (1) is one for converting specified addresses composedof eight bits corresponding to input gray level data 0 to 255 in theaddress generating circuit 12. Specifically, the 8-bit specifiedaddresses used as the basis (base addresses Oadr) corresponding to theinput gray level data 0 to 255 are converted. A specified addresscomposed of upper two bits corresponding to each of the pixels G₀, G₁,G₂ and G₃ in the dither matrix is not converted. FIG. 5 shows therelationship between the base addresses Oadr (the input gray level data)0 to 255 and 8-bit specified addresses obtained by the conversion Sadrand the relationship between the specified addresses Sadr and outputgray level data (total gray level data).

The graph line a is a graph indicating the original input graylevel-output gray level data in a case where input gray levels are usedto enter the axis of abscissa and output gray levels are used to enterthe axis of ordinate. A straight line LO is a graph indicating therelationship between the base addresses Oadr and the specified addressesSadr in a case where a one-to-one correspondence exists between the baseaddresses Oadr and the specified addresses Sadr. When the designatedcopy density is higher than the reference copy density, the relationshipbetween the base addresses Oadr and the specified addresses Sadr isconverted into the relationship indicated by, for example, a straightline LH. On the other hand, when the designated copy density is lowerthan the reference copy density, the relationship between the baseaddresses Oadr and the specified addresses Sadr is converted into therelationship indicated by, for example, a straight line LL.

Output gray level data corresponding to Point A of the base addressesOadr (the input gray level data) is OD when the relationship between thebase addresses Oadr and the specified addresses Sadr is indicated by thestraight line LO, is ODH (>OD) when the relationship between the baseaddresses Oadr and the specified addresses Sadr is indicated by thestraight line LH, and is ODL (<OD) when the relationship between thebase addresses Oadr and the specified addresses Sadr is indicated by thestraight line LL.

A formula for address conversion is given by the following expression(1):

    Sadr=Oadr+Sft                                              (1)

The value of Sft is changed depending on the copy density designated inthe operating section, to obtain an output gray level corresponding tothe designated copy density. The value of Sft corresponding to thedesignated copy density is previously determined, and is stored in thedata ROM 11 or the other ROM (not shown). Sft is so determined, if thedesignated copy density is higher than the reference copy density, thatit takes a positive value and that the higher the designated copydensity is, the larger its absolute value is. On the other hand, Sft isso determined, if the designated copy density is lower than thereference copy density, that it takes a negative value and that thelower the designated copy density is, the larger its absolute value is.

It is assumed that one type of input gray level-output gray level datacorresponding to a developing color and the type of document image outof a plurality of types of input gray level-output gray level datastored in the data ROM 11 are transferred to the table memory 13 fromthe data ROM 11 by the CPU 10, and the value of Sft corresponding to thedesignated copy density is sent to the address generating circuit 12.When input gray level data and an pixel position signal are sent to theaddress generating circuit 12, a specified address Sadr is found by theforegoing expression (1). Two bits corresponding to the pixel positionsignal are added to the specified address Sadr on the side of its mostsignificant digit, to generate an address specifying signal. The addressspecifying signal is outputted from the address generating circuit 12.

However, specified addresses obtained by the conversion Sadrcorresponding to input gray level data, whose values are larger than themaximum value 255 of the base addresses Oadr by the foregoing expression(1), are fixed to the maximum value 255. On the other hand, specifiedaddresses obtained by the conversion Sadr corresponding to input graylevel data, whose values are smaller than the minimum value 0 of thebase addresses Oadr, are fixed to the minimum value 0. When the addressspecifying signal is outputted from the address generating circuit 12,data at a corresponding address in the table memory 13 is outputted asoutput gray level data.

For example, if the designated copy density is higher than the referencecopy density and the value of Sft corresponding to the designated copydensity is +15, the specified addresses Sadr corresponding to the baseaddresses Oadr (the input gray level data) 0 to 255 become 15 to 270 bythe foregoing expression (1). The specified addresses Sadr correspondingto the input gray level data 241 to 255, whose values are larger than255, are set to the maximum value 255 of the base addresses Oadr.

The relationship between the input gray level data and the output graylevel data in a case where the value of Sft corresponding to thedesignated copy density is +15 is indicated by the graph line b in FIG.6. Specifically, the graph line b is a graph line obtained by shiftingthe graph line a indicating the original input gray level-output graylevel data by a predetermined amount to the left.

On the other hand, if the designated copy density is lower than thereference copy density and the value of Sft corresponding to thedesignated copy density is -15, the specified addresses Sadrcorresponding to the base addresses Oadr (the input gray level data) 0to 255 become -15 to 240 by the foregoing expression (1). The specifiedaddresses Sadr corresponding to the input gray level data 0 to 14, whosevalues are smaller than 0, are set to the minimum value 0 of the baseaddresses Oadr.

The relationship between the input gray level data and the output graylevel data in a case where the value of Sft corresponding to thedesignated copy density is -15 is indicated by the graph line c in FIG.6, Specifically, the graph line c is a graph line obtained by shiftingthe graph line a indicating the original input gray level-output graylevel data by a predetermined amount to the right.

(I-2) First method (2)

The first method (2) is one for converting transfer destinationaddresses in transferring input gray level-output gray level data to thetable memory 13 from the data ROM 11 without converting specifiedaddresses in the address generating circuit 12. In the first method (2),an address specifying signal used as the basis corresponding to a pixelin the dither matrix which corresponds to input gray level data and apixel position signal is outputted from the address generating circuit12.

The transfer of the input gray level-output gray level data from thedata ROM 11 to the table memory 13 is performed in the following manner.Specifically, one type of input gray level-output gray level datacorresponding to a developing color and the type of document image outof a plurality of types of input gray level-output gray level datastored in the data ROM 11 are selected. In transferring the selectedinput gray level-output gray level data from the data ROM 11 to thetable memory 13, the transfer destination addresses are converted.

Each of the transfer destination addresses comprises two upper bits,which express values "00", "01", "10" and "11", representing each of theareas TE₀, TE₁, TE₂ and TE₃ in the table memory 13 and eight lower bitsrepresenting addresses 0 to 255 in each of the areas TE₀, TE₁, TE₂ andTE₃. A base transfer destination address OTadr composed of the lowereight bits representing the addresses 0 to 255 in each of the areas TE₀,TE₁, TE₂ and TE₃ is converted into a new transfer destination addressNTadr by the following expression (2). A transfer destination addresscomposed of the upper two bits representing each of the areas TE₀, TE₁,TE₂ and TE₃ in the table memory 13 is not converted.

    NTadr=OTadr-Sft                                            (2)

In this case, the value of Sft corresponding to the designated copydensity is previously determined and stored in the data ROM 11 or theother ROM. Base transfer destination addresses OTadr corresponding torespective input gray level-output gray level data in each of the areasTE₀, TE₁, TE₂ and TE₃ corresponding to the pixels G₀, G₁, G₂ and G₃ inthe dither matrix are converted into new transfer destination addressesNTadr on the basis of the foregoing expression (2) from the value of Sftcorresponding to the designated copy density. The respective data aretransferred to the new transfer destination addresses NTadr in each ofthe areas TE₀, TE₁, TE₂ and TE₃ corresponding to the pixels G₀, G₁, G₂and G₃ in the dither matrix in the table memory 13.

When the values of new transfer destination addresses NTadr are outsidethe range of 0 to 255, however, data corresponding to the new transferdestination addresses NTadr are not transferred. In addition, when suchconversion of the transfer destination addresses is made, a blankportion where there exist no data to be transferred occurs in a portionassigned small addresses or a portion assigned large addresses in eachof the areas TE₀, TE₁, TE₂ and TE₃ in the table memory 13. If the blankportion occurs in the portion assigned large addresses in each of theareas TE₀, TE₁, TE₂ and TE₃ in the table memory 13, therefore, the samedata as data transferred to the largest new transfer destination addressNTadr out of the new transfer destination addresses NTadr in the area istransferred to the blank portion in the area. On the other hand, if theblank portion occurs in the portion assigned small addresses in each ofthe areas TE₀, TE₁, TE₂ and TE₃ in the table memory 13, the same data asdata transferred to the smallest new transfer destination address NTadrout of the new transfer destination addresses NTadr in the area istransferred to the blank portion in the area.

For example, if the designated copy density is higher than the referencecopy density and the value of Sft corresponding to the designated copydensity is +15, the base transfer destination addresses OTadr 0 to 255are converted into new transfer designation addresses NTadr -15 to 240by the foregoing expression (2).

Data excluding data corresponding to the new transfer destinationaddresses NTadr whose values are smaller than 0 are transferred to thecorresponding new transfer destination addresses NTadr in the areas TE₀,TE₁, TE₂ and TE₃ in the table memory 13. In the above described example,therefore, data corresponding to the new transfer destination addressesNTadr 0 to 240 are transferred to the corresponding new transferdestination addresses in the areas TE₀, TE₁, TE₂ and TE₃ in the tablememory 13. Data corresponding to the new transfer destination addressesNTadr -15 to -1 are not transferred.

Furthermore, when such transfer is performed, the blank portion occursin the portion assigned large addresses in each of the areas TE₀, TE₁,TE₂ and TE₃ in the table memory 13. Accordingly, the same data as datatransferred to the largest new transfer destination address NTadr (240in the above described example) out of the new transfer destinationaddresses NTadr in the area is transferred to the blank portion in thearea. If an address specifying signal is outputted from the addressgenerating circuit 12 after the transfer is performed, datacorresponding to an address specified is outputted from the table memory13. The relationship between the input gray level data and the outputgray level data in a case where the value of Sft corresponding to thedesignated copy density is +15 is indicated by the graph line b in FIG.6.

On the other hand, if the designated copy density is lower than thereference copy density and the value of Sft corresponding to thedesignated copy density is -15, the base transfer destination addressesOTadr 0 to 255 corresponding to the respective data are converted intonew transfer destination addresses NTadr 15 to 270 by the foregoingexpression (2).

Data excluding data corresponding to the new transfer destinationaddresses NTadr whose values are larger than 255 are transferred to thecorresponding new transfer destination addresses NTadr in the areas TE₀,TE₁, TE₂ and TE₃ in the table memory 13. In the above described example,therefore, data corresponding to the new transfer destination addressesNTadr 15 to 255 are transferred to the corresponding new transferdestination addresses in the areas TE₀, TE₁, TE₂ and TE₃ in the tablememory 13. Data corresponding to the new transfer destination addressesNTadr 256 to 270 are not transferred.

Furthermore, when such transfer is performed, the blank portion occursin the portion assigned small addresses in each of the areas TE₀, TE₁,TE₂ and TE₃ in the table memory 13. Accordingly, the same data as datatransferred to the smallest new transfer destination address NTadr (15in the above described example) out of the new transfer destinationaddresses NTadr in the area is transferred to the blank portion in thearea. If an address specifying signal is outputted from the addressgenerating circuit 12 after the transfer is performed, datacorresponding to an address specified is outputted from the table memory13. The relationship between the input gray level data and the outputgray level data in a case where the value of Sft corresponding to thedesignated copy density is -15 is indicated by the graph line c in FIG.6.

If the input gray level-output gray level characteristics obtained bythe above described first method which are indicated by the graph linesb or c in FIG. 6 are compared with the original input gray level-outputgray level characteristics which are indicated by the graph line a inFIG. 6, the value of an output gray level at a point on thecharacteristic curve b or c is unchanged from the value of an outputgray level at a corresponding point on the characteristic curve a.Accordingly, the relationship between the output gray level data and thedensity at which printing is actually done in the original input graylevel-output gray level characteristics can be maintained. Therefore,the relationship between the input gray level data and the density atwhich printing is actually done can be made linear. In addition, therange of output gray levels is not extremely decreased, as compared withthat in a method of merely adding or subtracting an offset value to orfrom an output gray level found by the original input gray level-outputgray level data.

(II) Second Method

As shown in FIG. 8, the second method is one for rotating, in a casewhere input gray levels are used to enter the axis of abscissa, outputgray levels are used to enter the axis of ordinate, and the originalinput gray level-output gray level characteristics are indicated by agraph line a, the graph line a through a predetermined angle in aclockwise direction or in a counterclockwise direction around a point,which corresponds to the input gray level 255, on the graph line adepending on the designated copy density, to convert the original inputgray level-output gray level characteristics into characteristicscorresponding to the designated copy density (a graph line b or c).There are two methods used for carrying out the second method.

(II-1) Second Method (1)

The second method (1) is one for converting specified addresses composedof eight bits (base addresses Oadr) corresponding to input gray leveldata 0 to 255 in the address generating circuit 12. A specified addresscomposed of two upper bits corresponding to each of the pixels G₀, G₁,G₂ and G₃ in the dither matrix is not converted. FIG. 7 shows therelationship between the base addresses Oadr 0 to 255 and specifiedaddresses obtained by the conversion Sadr and the relationship betweenthe specified addresses Sadr and output gray level data (total graylevel data).

The graph line a is a graph indicating the original input graylevel-output gray level data in a case where input gray levels are usedto enter the axis of abscissa and output gray levels are used to enterthe axis of ordinate. A straight line LO is a graph indicating therelationship between the base addresses Oadr and the specified addressesSadr in a case where a one-to-one correspondence exists between the baseaddresses Oadr and the specified addresses Sadr. When the designatedcopy density is higher than the reference copy density, the relationshipbetween the base addresses Oadr and the specified addresses Sadr isconverted into the relationship indicated by a straight line LH. On theother hand, when the designated copy density is lower than the referencecopy density, the relationship between the base addresses Oadr and thespecified addresses Sadr is converted into the relationship indicated bya straight line LL.

Output gray level data corresponding to Point A of the base addressesOadr (the input gray level data) is OD when the relationship between thebase addresses Oadr and the specified addresses Sadr is indicated by thestraight line LO, is ODH (>OD) when the relationship between the baseaddresses Oadr and the specified addresses Sadr is indicated by thestraight line LH, and is ODL (<OD) when the relationship between thebase addresses Oadr and the specified addresses Sadr is indicated by thestraight line LL.

A formula for address conversion is given by the following expression(3):

    Sadr=255-{Gain (255-Oadr)+Offset}                          (3)

In the foregoing expression (3), the slope of the straight line LO(Sadr/Oadr) can be adjusted by adjusting the value of Gain. In thiscase, the slope of the straight line LO is adjusted around a point,which corresponds to the input gray level 255, on the straight line LO,so that the value of Offset is always 0.

The value of Gain is changed depending on the copy density designated inthe operating section, to obtain an output gray level corresponding tothe designated copy density. The value of Gain corresponding to thedesignated copy density is previously determined, and is stored in thedata ROM 11 or the other ROM (not shown). Gain is so determined, if thedesignated copy density is higher than the reference copy density, thatit takes a value smaller than 1 and that the higher the designated copydensity is, the smaller the value is. On the other hand, Gain is sodetermined, if the designated copy density is lower than the referencecopy density, that it takes a value larger than 1 and that the lower thedesignated copy density is, the larger the value is.

It is assumed that one type of input gray level-output gray level datacorresponding to a developing color and the type of document image outof a plurality of types of input gray level-output gray level datastored in the data ROM 11 are transferred to the table memory 13 fromthe data ROM 11 by the CPU 10, and the value of Gain corresponding tothe designated copy density is sent to the address generating circuit 12by the CPU 10. When input gray level data and a pixel position signalare sent to the address generating circuit 12, a specified address Sadris found by the foregoing expression (3). Two bits corresponding to thepixel position signal are added to the specified address Sadr on theside of its most significant digit, to generate an address specifyingsignal. The address specifying signal is outputted from the addressgenerating circuit 12.

However, specified addresses obtained by the conversion Sadrcorresponding to input gray level data, whose values are smaller thanthe minimum value 0 of the base addresses Oadr by the foregoingexpression (3), are fixed to the minimum value 0. When the addressspecifying signal is outputted from the address generating circuit 12,data at a corresponding address in the table memory 13 is outputted asoutput gray level data.

For example, if the designated copy density is higher than the referencecopy density and the value of Gain corresponding to the designated copydensity is 0.9, the relationship between the input gray level data andthe output gray level data is indicated by the graph line b in FIG. 8.Specifically, the graph line b is a graph line obtained by rotating thegraph line a indicating the original input gray level-output gray leveldata stored in the data ROM 11 through a predetermined angle in aclockwise direction around a point, which corresponds to the input graylevel 255, on the graph line a. On the other hand, if the designatedcopy density is lower than the reference copy density and the value ofGain corresponding to the designated copy density is 1.1, therelationship between the input gray level data and the output gray leveldata is indicated by the graph line c in FIG. 8. Specifically, the graphline c is a graph line obtained by rotating the graph line a indicatingthe original input gray level-output gray level data stored in the dataROM 11 through a predetermined angle in a counterclockwise directionaround a point, which corresponds to the input gray level 255, on thegraph line a.

(II-2) Second Method (2)

The second method (2) is one for converting transfer destinationaddresses in transferring input gray level-output gray level data to thetable memory 13 from the data ROM 11 without converting specifiedaddresses in the address generating circuit 12. In this second method(2), an address specifying signal used as the basis corresponding to apixel in the dither matrix which corresponds to input gray level dataand a pixel position signal is outputted from the address generatingcircuit 12.

The transfer of the input gray level-output gray level data from thedata ROM 11 to the table memory 13 is performed in the following manner.Specifically, one type of input gray level-output gray level datacorresponding to a developing color and the type of document image outof a plurality of types of input gray level-output gray level datastored in the data ROM 11 are selected. In transferring the selectedinput gray level-output gray level data from the data ROM 11 to thetable memory 13, the transfer destination addresses are converted.

Each of the transfer destination address comprises two upper bits, whichexpress values "00", "01", "10" and "11", representing each of the areasTE₀, TE₁, TE₂ and TE₃ in the table memory 13 and eight lower bitsrepresenting addresses 0 to 255 in each of the areas TE₀, TE₁, TE₂ andTE₃. A base transfer destination address OTadr composed of the lowereight bits representing the addresses 0 to 255 in each of the areas TE₀,TE₁, TE₂ and TE₃ is converted into a new transfer destination addressNTadr by the following expression (4). A transfer destination addresscomposed of the upper two bits representing each of the areas TE₀, TE₁,TE₂ and TE₃ in the table memory 13 is not converted.

    NTadr={OTadr+255 (Gain-1)+Offset) ÷Gain                (4)

The value of Offset is 0. Also in this case, the value of Gaincorresponding to the designated copy density is previously determinedand stored in the data ROM 11 or the other ROM. Base transferdestination addresses OTadr corresponding to respective input graylevel-output gray level data in each of the areas TE₀, TE₁, TE₂ and TE₃corresponding to the pixels G₀, G₁, G₂ and G₃ in the dither matrix areconverted into new transfer destination addresses NTadr on the basis ofthe foregoing expression (4) from the value of Gain corresponding to thedesignated copy density. The respective data are transferred to the newtransfer destination addresses NTadr in each of the areas TE₀, TE₁, TE₂and TE₃ corresponding to the pixels G₀, G₁, G₂ and G₃ in the dithermatrix in the table memory 13.

The values of new transfer destination addresses NTadr may, in somecases, be smaller than 0. However, data corresponding to the newtransfer destination addresses NTadr are not transferred. In addition,when such conversion of the transfer destination addresses is made, ablank portion where there exist no data to be transferred may, in somecases, occur in a portion assigned small addresses in each of the areasTE₀, TE₁, TE₂ and TE₃ in the table memory 13. If the blank portionoccurs in the portion assigned small addresses in each of the areas TE₀,TE₁, TE₂ and TE₃ in the table memory 13, therefore, the same data asdata transferred to the smallest new transfer destination address NTadrout of the new transfer destination addresses NTadr in the area istransferred to the blank portion in the area.

If an address specifying signal is outputted from the address generatingcircuit 12 after the transfer is performed, data corresponding to anaddress specified is outputted from the table memory 13. Therelationship between the input gray level data and the output gray leveldata in a case where the value of Gain corresponding to the designatedcopy density is 0.9 is indicated by the graph line b in FIG. 8. Therelationship between the input gray level data and the output gray leveldata in a case where the value of Gain corresponding to the designatedcopy density is 1.1 is indicated by the graph line c in FIG. 8.

If the input gray level-output gray level characteristics obtained bythe above described second method are compared with the original inputgray level-output gray level characteristics, the value of an outputlevel at a point on the characteristic curve is slightly changed fromthe value of an output gray level at a corresponding point on theoriginal characteristic curve in a low density portion. However, therange of input gray levels which can be reproduced is not decreased, ascompared with that in the first method.

(III) Third Method

As shown in FIG. 10, the third method is one for so rotating, in a casewhere input gray levels are used to enter the axis of abscissa, outputgray levels are used to enter the axis of ordinate, and the originalinput gray level-output gray level characteristics are indicated by agraph line a, the graph line a through a predetermined angle in aclockwise direction or in a counterclockwise direction around a point,which corresponds to the input gray level 0, on the graph line adepending on the designated copy density, to convert the original inputgray level-output gray level characteristics into characteristicscorresponding to the designated copy density (a graph line b or c).There are two methods used for carrying out the third method.

(III-1) Third Method (1)

The third method (1) is one for converting specified addresses composedof eight bits (base addresses Oadr) corresponding to input gray leveldata 0 to 255 in the address generating circuit 12. A specified addresscomposed of two upper bits corresponding to each of the pixels G₀, G₁,G₂ and G₃ in the dither matrix is not converted. FIG. 9 shows therelationship between the base addresses Oadr 0 to 255 and specifiedaddresses obtained by the conversion Sadr and the relationship betweenthe specified addresses Sadr and output gray level data (total graylevel data).

The graph line a is a graph indicating the original input graylevel-output gray level data in a case where input gray levels are usedto enter the axis of abscissa and output gray levels are used to enterthe axis of ordinate. A straight line LO is a graph indicating therelationship between the base addresses Oadr and the specified addressesSadr in a case where a one-to-one correspondence exists between the baseaddresses Oadr and the specified addresses Sadr. When the designatedcopy density is higher than the reference copy density, the relationshipbetween the base addresses Oadr and the specified addresses Sadr isconverted into the relationship indicated by a straight line LH. On theother hand, when the designated copy density is lower than the referencecopy density, the relationship between the base addresses Oadr and thespecified addresses Sadr is converted into the relationship indicated bya straight line LL.

Output gray level data corresponding to Point A of the base addressesOadr (the input gray level data) is OD when the relationship between thebase addresses Oadr and the specified addresses Sadr is indicated by thestraight line LO, is ODH (>OD) when the relationship between the baseaddresses Oadr and the specified addresses Sadr is indicated by thestraight line LH, and is ODL (<OD) when the relationship between thebase addresses Oadr and the specified addresses Sadr is indicated by thestraight line LL.

A formula for address conversion is given by the following expression(5):

    Sadr=255-{Gain (255-Oadr)+Offset}                          (5)

In the foregoing expression (5), the slope of the straight line LO(Sadr/Oadr) can be adjusted by adjusting the value of Gain. In thiscase, the slope of the straight line LO is adjusted around a point,which corresponds to the input gray level 0, on the straight line LO, sothat the value of Offset is so adjusted that the value of {255.Gain+Offset] is 255.

The values of Gain and Offset are changed depending on the copy densitydesignated in the operating section, to obtain an output gray levelcorresponding to the designated copy density. The values of Gain andOffset corresponding to the designated copy density are previouslydetermined, and are stored in the data ROM 11 or the other ROM (notshown). Gain is so determined, if the designated copy density is higherthan the reference copy density, that it takes a value larger than 1 andthat the higher the designated Copy density is the larger the value is.On the other hand. Gain is so determined, if the designated copy densityis lower than the reference copy density, that it takes a value smallerthan 1 and that the lower the designated copy density is, the smallerthe value is.

It is assumed that one type of input gray level-output gray level datacorresponding to a developing color and the type of document image outof a plurality of types of input gray level-output gray level datastored in the data ROM 11 are transferred to the table memory 13 fromthe data ROM 11 by the CPU 10, and the values of Gain and Offsetcorresponding to the designated copy density are sent to the addressgenerating circuit 12 by the CPU 10. When input gray level data and apixel position signal are sent to the address generating circuit 12, aspecified address Sadr is found by the foregoing expression (5). Twobits corresponding to the pixel position signal are added to thespecified address Sadr on the side of its most significant digit, togenerate an address specifying signal. The address specifying signal isoutputted from the address generating circuit 12.

However, specified addresses Sadr corresponding to input gray leveldata, whose values are larger than the maximum value 255 of the baseaddresses Oadr by the foregoing expression (5), are fixed to the maximumvalue 255. When the address specifying signal is outputted from theaddress generating circuit 12, data at a corresponding address in thetable memory 13 is outputted as output gray level data.

For example, if the designated copy density is higher than the referencecopy density and the values of Gain and Offset corresponding to thedesignated copy density are respectively 1.1 and -25.5, the relationshipbetween the input gray level data and the output gray level data isindicated by the graph line b in FIG. 10. Specifically, the graph line bis a graph line obtained by rotating the graph line a indicating theoriginal input gray level-output gray level data stored in the data ROM11 through a predetermined angle in a counterclockwise direction arounda point, which corresponds to the input gray level 0, on the graph linea. On the other hand, if the designated copy density is lower than thereference copy density and the values of Gain and Offset correspondingto the designated copy density are respectively 0.9 and +25.5, therelationship between the input gray level data and the output gray leveldata is indicated by the graph line c in FIG. 10. Specifically, thegraph line c is a graph line obtained by rotating the graph line aindicating the original input gray level-output gray level data storedin the data ROM 11 through a predetermined angle in a clockwisedirection around a point, which corresponds to the input gray level 255,on the graph line a.

(III-2) Third Method (2)

The third method (2) is one for converting transfer destinationaddresses in transferring input gray level-output gray level data to thetable memory 13 from the data ROM 11 without converting specifiedaddresses in the address generating circuit 12. In this third method(2), an address specifying signal used as the basis corresponding to apixel in the dither matrix which corresponds to input gray level dataand a pixel position signal is outputted from the address generatingcircuit 12.

The transfer of the input gray level-output gray level data from thedata ROM 11 to the table memory 13 is performed in the following manner.Specifically, one type of input gray level-output gray level datacorresponding to a developing color and the type of document image outof a plurality of types of input gray level-output gray level datastored in the data ROM 11 are selected. In transferring the selectedinput gray level-output gray level data from the data ROM 11 to thetable memory 13, the transfer destination addresses are converted.

Each of the transfer destination addresses comprises two upper bits,which express values "00", "01", "10" and "11", representing each of theareas TE₀, TE₁, TE₂ and TE₃ in the table memory 13 and eight lower bitsrepresenting addresses 0 to 255 in each of the areas TE₀, TE₁, TE₂ andTE₃. A base transfer destination address OTadr composed of the lowereight bits representing the addresses 0 to 255 in each of the areas TE₀,TE₁, TE₂ and TE₃ is converted into a new transfer destination addressNTadr by the following expression (6). A transfer destination addresscomposed of the upper two bits representing each of the areas TE₀, TE₁,TE₂ and TE₃ in the table memory 13 is not converted.

    NTadr={OTadr+255 (Gain-1)+Offset} ÷Gain                (6)

Also in this case, the values of Gain and Offset corresponding to thedesignated copy density are previously determined and stored in the dataROM 11 or the other ROM. Base transfer destination addresses OTadrcorresponding to respective input gray level-output gray level data ineach of the areas TE₀, TE₁, TE₂ and TE₃ corresponding to the pixels G₀,G₁, G₂ and G₃ in the dither matrix are converted into new transferdestination addresses NTadr on the basis of the foregoing expression (6)from the value of Sft corresponding to the designated copy density. Therespective data are transferred to the new transfer destinationaddresses NTadr in each of the areas TE₀, TE₁, TE₂ and TE₃ correspondingto the pixels G₀, in G₁, G₂ and G₃ in the dither matrix in the tablememory 13.

When the values of new transfer destination addresses NTadr are largerthan 255, however, data corresponding to the new transfer destinationaddress NTadr are not transferred. In addition, when such conversion ofthe transfer destination addresses is made, a blank portion may, in somecases, occur in a portion assigned large addresses in each of the areasTE₀, TE₁, TE₂ and TE₃ in the table memory 13. If the blank portionoccurs in the portion assigned large addresses in each of the areas TE₀,TE₁, TE₂ and TE₃ in the table memory 13, therefore, the same data asdata transferred to the largest new transfer destination address NTadrout of the new transfer destination addresses NTadr in the area istransferred to the blank portion in the area.

If an address specifying signal is outputted from the address generatingcircuit 12 after the transfer is performed, data corresponding to anaddress specified is outputted from the table memory 13. Therelationship between the input gray level data and the output gray leveldata in a case where the values of Gain and Offset corresponding to thedesignated copy density are respectively 1.1 and -25.5 is indicated bythe graph line b in FIG. 10. The relationship between the input graylevel data and the output gray level data in a case where the values ofGain and Offset corresponding to the designated copy density arerespectively 0.9 and +25.5 is indicated by the graph line c in FIG. 10.

If the input gray level-output gray level characteristics obtained bythe above described third method are compared with the original inputgray level-output gray level characteristics, the value of an outputgray level at a point on the characteristic curve is slightly changedfrom the value of an output gray level at a corresponding point on theoriginal characteristic curve in a high density portion. However, therange of input gray levels which can be reproduced is not decreased, ascompared with that in the first method.

In the third method, the expression (5) may be replaced with thefollowing expression (7), and the expression (6) may be replaced withthe following expression (8):

    Aadr=Gain×Oadr                                       (7)

    NTadr=(1/Gain)×OTadr                                 (8)

(IV) Fourth Method

The fourth method is a method using the above described second methodwhen the copy density designated in the operating section is higher thanthe reference copy density and using the above described third methodwhen the copy density designated in the operating section is lower thanthe reference copy density.

Specifically, when the designated copy density is higher than thereference copy density, the second method is so used, in a case wherethe original input gray level-output gray level characteristics areindicated by the graph line a in FIG. 8, that the original input graylevel-output gray level characteristics are converted intocharacteristics (the graph line b) obtained by rotating the graph line athrough a predetermined angle in a clockwise direction around a point,which corresponds to the input gray level 255, on the graph line a inconformity with the designated copy density. On the other hand, when thedesignated copy density is lower than the reference copy density, thethird method is so used, in a case where the original input graylevel-output gray level characteristics are indicated by the graph linea in FIG. 10, that the original input gray level-output gray levelcharacteristics are converted into characteristics (the graph line c)obtained by rotating the graph line a through a predetermined angle in aclockwise direction around a point, which corresponds to the input graylevel 0, on the graph line a in conformity with the designated copydensity.

There are two methods used for carrying out the fourth method, that is,a method of converting specified addresses in the address generatingcircuit 12 and a method of converting transfer destination addresses intransferring input gray level-output gray level data to the table memory13 from the data ROM 11 without converting specified addresses in theaddress generating circuit 12.

In the method of converting the specified addresses in the addressgenerating circuit 12, the specified addresses are so converted thatoutput gray levels are higher in the above described second method (1)when the designated copy density is higher than the reference copydensity, while being so converted that output gray levels are lower inthe above described third method (2) when the designated copy density islower than the reference copy density. In this case, as a formula forconverting the specified address, the expression (3) common to thesecond and third methods is used.

In the method of converting the transfer destination addresses intransferring input gray level-output gray level data from the data ROM11 to the table memory 13, the transfer destination addresses are soconverted that output gray levels are higher in the above describedsecond method (2) when the designated copy density is higher than thereference copy density, while being so converted that output gray levelsare lower in the above described third method (2) when the designatedcopy density is lower than the reference copy density. In this case, asa formula for converting transfer destination addresses, the expression(4) common to the second and third methods is used.

The fourth method has the advantage that the change of a document imagecorresponding to the designated copy density is easily recognized by aviewer because a low density portion of the document image is madeparticularly dark in the recorded image when the designated copy densityis higher than the reference copy density, while a high density portionof the document image is made particularly light in the recorded imagewhen the designated copy density is lower than the reference copydensity.

A table memory having a capacity capable of storing four types of inputgray level-output gray level data corresponding to four developingcolors may be used as the table memory 13, and the input graylevel-output gray level data corresponding to the four developing colorswhich correspond to the type of the document image out of a plurality oftypes of input gray level-output gray level data stored in the data ROM11 may be transferred. In this case, a signal representing a developingcolor is sent to the address generating circuit 12 from the CPU 10, sothat a 12-bit address specifying signal including the signalrepresenting a developing color is outputted from the address generatingcircuit 12.

(V) Fifth Method

As shown in FIG. 12 or 14, the fifth method is one for shifting, in acase where input gray levels are used to enter the axis of abscissa,output gray levels are used to enter the axis of ordinate, and theoriginal input gray level-output gray level characteristics (totaloutput gray levels) are indicated by a graph line a, the graph line a tothe left or right depending on the designated copy density, to convertthe original input gray level-output gray level characteristics intocharacteristics corresponding to the designated copy density, andfinding output gray levels relative to input gray levels at which thereexist no output gray level data as the result of the above describedshifting by a predetermined operation.

(V-1) Description is now made of the idea of the fifth method by takingas an example a case where the designated copy density is lower than thereference copy density.

FIGS. 11 and 12 show input gray level-output gray level characteristicsin a case where input gray levels are used to enter the axis of abscissaand output gray levels are used to enter the axis of ordinate. A graphline a shown in FIGS. 11 and 12 indicates the original input graylevel-output gray level characteristics. When the designated copydensity is lower than the reference copy density, the original inputgray level-output gray level characteristics are converted intocharacteristics obtained by shifting the graph line a indicating theoriginal input gray level-output gray level characteristics to theright, as indicated by a graph line c1 in FIG. 11. The amount of theshifting is set to a value corresponding to the designated copy density.If the graph line a indicating the original input gray level-output graylevel characteristics is merely shifted to the right, no input graylevel-output gray level data exist in a portion at low input graylevels. Accordingly, input gray level-output gray level data in theportion are indicated by a graph line c20 in FIG. 12 which is a straightline having a gradient θ of, for example, 45°. The input gray levels arethus converted on the basis of input gray level-output gray level datac2 found.

(V-1-1) Description is made of a case where the designated copy densityis lower than the reference copy density using the idea of convertingspecified addresses. The original input gray level-output gray leveldata indicated by the graph line a in FIGS. 11 and 12 is shown in Table2.

                  TABLE 2                                                         ______________________________________                                                   BASE       OUTPUT GRAY  TOTAL                                      INPUT GRAY ADDRESS    LEVEL DATA   GRAY                                       LEVEL DATA (Oadr)     G.sub.0                                                                             G.sub.1                                                                            G.sub.2                                                                           G.sub.3                                                                           LEVEL                                ______________________________________                                        0          0          9     9    9   9   36                                   1          1          10    9    9   9   37                                   2          2          10    10   9   9   38                                   3          3          10    10   9   10  39                                   .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    219        219        64    64   63  64  255                                  .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    255        255        64    64   63  64  255                                  ______________________________________                                    

The characteristics cl obtained by shifting the graph line a indicatingthe original input gray level-output gray level characteristics shown inFIG. 11 to the right are obtained by converting base addresses Oadr intospecified addresses Sadr using the same expression (9) as the foregoingexpression (1):

    Sadr=Oadr+Sft                                              (9)

If the value of Sft corresponding to the characteristics c1 shown inFIG. 11 is -25, the specified addresses Sadr corresponding to the baseaddresses Oadr 0 to 255 are converted into -25 to 230 by the expression(9). If the specified addresses are thus converted by setting the valueof Sft to -25, the values of the specified addresses obtained by theconversion Sadr corresponding to the input gray level data 0 to 24become smaller than the minimum value 0 of the base addresses Oadr.Consequently, there exist no output gray level data corresponding to theinput gray level data 0 to 24.

In such a case, it is considered that the specified addresses Sadrcorresponding to the input gray level data 0 to 24 are set to theminimum value 0 of the base addresses Oadr. Consequently, output graylevel data corresponding to the input gray level data 0 to 24 are thesame as output gray level data corresponding to the input gray leveldata 25. The specified addresses Sadr and the output gray level datacorresponding to the input gray level data in such a case are as shownin Table 3. In addition, the output gray level data corresponding to theinput gray level data 0 to 24 are graphically indicated by a straightportion c10 in FIG. 11.

                  TABLE 3                                                         ______________________________________                                                   SPECIFIED  OUTPUT GRAY  TOTAL                                      INPUT GRAY ADDRESS    LEVEL DATA   GRAY                                       LEVEL DATA (Sadr)     G.sub.0                                                                             G.sub.1                                                                            G.sub.2                                                                           G.sub.3                                                                           LEVEL                                ______________________________________                                        0          0          9     9    9   9   36                                   1          0          9     9    9   9   36                                   .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    24         0          9     9    9   9   36                                   25         0          9     9    9   9   36                                   26         1          10    9    9   9   37                                   .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    234        219        64    64   63  64  255                                  .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    255        230        64    64   63  64  255                                  ______________________________________                                    

In the fifth method, the output gray level data corresponding to theinput gray level data which correspond to the specified addresses Sadrwhose values are smaller than 0 (the input gray levels 0 to 24 in theabove described example) are found on the basis of a predetermined rulefrom total output gray level data (36 in the above descried example)corresponding to the input gray level data which corresponds to thespecified address Sadr whose value is 0 (the input gray level 25 in theabove described example). This rule is so determined that total graylevels corresponding to the input gray level data 0 to 24 are decreasedone at a time in descending order of input gray levels from the totalgray level 36 corresponding to the input gray level data 25.

Such a rule is not limited to the foregoing rule. That is, other rulesmay be used. For example, the rule may be so determined that total graylevels corresponding to the input gray level data 0 to 24 are decreaseda predetermined value other than one at a time in descending order ofinput gray levels from the total gray level 36 corresponding to theinput gray level data 25. In addition, the rule may be so determinedthat total gray levels corresponding to the input gray level data 0 to24 are decreased a predetermined value at a time in descending order ofinput gray levels and every a predetermined number of input gray levelsfrom the total gray level 36 corresponding to the input gray level data25.

Table 4 shows output gray levels corresponding to the input gray leveldata 0 to 24 so found that the total gray levels corresponding to theinput gray level data 0 to 24 are decreased one at a time in descendingorder of input gray levels from the total gray level 36 corresponding tothe input gray level data 25. In addition, the relationship between theinput gray level data and the output gray level data in a case whereinput gray level-output gray level data corresponding to the input graylevels 0 to 24 are thus corrected is indicated by the graph line c2 inFIG. 12. This graph line c2 is obtained by shifting the graph line ashown in FIG. 12 by a predetermined amount to the right and indicatinginput gray level-output gray level characteristics corresponding to theinput gray levels 0 to 24 by a straight portion c20 which is a straightline passing through a point, which corresponds to the input gray leveldata 25, on a graph line obtained by the shifting and having a gradientθ of 45°.

                  TABLE 4                                                         ______________________________________                                                   SPECIFIED  OUTPUT GRAY  TOTAL                                      INPUT GRAY ADDRESS    LEVEL DATA   GRAY                                       LEVEL DATA (Sadr)     G.sub.0                                                                             G.sub.1                                                                            G.sub.2                                                                           G.sub.3                                                                           LEVEL                                ______________________________________                                        0          -25        0     0    2   9   11                                   1          -24        0     0    3   9   12                                   .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    8          -17        0     1    9   9   19                                   9          -16        0     2    9   9   20                                   .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    16         -9         0     9    9   9   27                                   17         -8         1     9    9   9   28                                   .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    .          .          .     .    .   .   .                                    24         -1         8     9    9   9   35                                   25         0          9     9    9   9   36                                   ______________________________________                                    

(V-1-2) Description is now made of processing actually performed.

In the actual processing, specified addresses are not converted in theaddress generating circuit 12. Specifically, an address specifyingsignal used as the basis corresponding to a pixel in the dither matrixwhich corresponds to input gray level data and a pixel position signalis outputted from the address generating circuit 12. The CPU 10 readsout input gray level-output gray level data corresponding to adeveloping color and the designated copy density from the data ROM 11and transfers the same to the RAM 14 on the basis of the developingcolor and the designated copy density.

Transfer destination addresses to the table memory 13 corresponding tothe input gray level-output gray level data transferred to the RAM 14are converted. This conversion of the transfer destination addresses isrepresented by the same expression (10) as the foregoing expression (2):

    NTadr=OTadr-Sft                                            (10)

The value of Sft corresponding to the designated copy density ispreviously determined and stored in the data ROM 11 or the other ROM.Base transfer destination addresses OTadr corresponding to respectiveinput gray level-output gray level data in each of the areas TE₀, TE₁,TE₂ and TE₃ corresponding to the pixels G₀, G₁, G₂ and G₃ in the dithermatrix are converted into new transfer destination addresses NTadr onthe basis of the foregoing expression (10) from the value of Sftcorresponding to the designated copy density. The respective data aretransferred to the new transfer destination addresses NTadr in each ofthe areas TE₀, TE₁, TE₂ and TE₃ corresponding to the pixels G₀, G₁, G₂and G₃ in the dither matrix in the table memory 13.

If the designated copy density is lower than the reference copy densityand the value of Sft corresponding to the designated copy density is-25, the base transfer destination addresses OTadr 0 to 255corresponding to the respective data are converted into new transferdestination addresses NTadr 25 to 280 by the foregoing expression (10).Data corresponding to the new transfer destination addresses NTadr whosevalues are larger than the maximum value 255 of the base transferdestination addresses OTadr are deleted.

By the conversion of the transfer destination addresses, blank transferdestination addresses at which there exist no data to be transferred(addresses 0 to 24 in the above described example) are generated in aportion assigned small addresses in each of the areas TE₀, TE₁, TE₂ andTE₃ in the table memory 13. Therefore, output gray level datacorresponding to the blank transfer destination addresses are generatedin the following manner.

The output gray level data corresponding to the blank transferdestination addresses are found on the basis of a predetermined rulefrom output gray level data corresponding to the minimum value of thenew transfer destination addresses NTadr. This rule is so determinedthat total gray levels corresponding to the respective blank transferdestination addresses are decreased one at a time in descending order ofblank transfer destination addresses from the total gray level (36 inthe above described example) corresponding to the minimum value of thenew transfer destination addresses NTadr (25 in the above describedexample). The output gray level data corresponding to the blank transferdestination addresses 0 to 24 are the same as the output gray level datacorresponding to the input gray level data 0 to 24 shown in Table 4.

If output gray level data corresponding to the whole of each of theareas TE₀, TE₁, TE₂ and TE₃ in the table memory 13 are thus generated inthe RAM 14, the data are transferred to the table memory 13 from the RAM14. Thereafter, if an address specifying signal is outputted from theaddress generating circuit 12, data is outputted from a correspondingaddress in the table memory 13.

(V-2) Description is made of a case where the designated copy density ishigher than the reference copy density.

A graph line a shown in FIGS. 13 and 14 indicates the original inputgray level-output gray level characteristics. When the designated copydensity is higher than the reference copy density, the original inputgray level-output gray level characteristics are converted intocharacteristics obtained by shifting the graph line a indicating theoriginal input gray level-output gray level characteristics to the left,as indicated by a graph line b1 in FIG. 13. The amount of the shiftingis set to a value corresponding to the designated copy density. If thegraph line a indicating the original input gray level-output gray levelcharacteristics is merely shifted to the left, no input graylevel-output gray level data exist in a portion at high input graylevels. Accordingly, input gray level-output gray level data in theportion are indicated by a graph line b20 in FIG. 14 which is a straightline having a gradient θ of 45°. The input gray levels are thusconverted on the basis of input gray level-output gray level data b2found.

(V-2-1) Description is made of a case where the designated copy densityis higher than the reference copy density using the idea of convertingspecified addresses.

If the designated copy density is higher than the reference copy densityand the value of Sft in the foregoing expression (9) is +15, specifiedaddresses Sadr corresponding to input gray level data 0 to 255 (baseaddresses Oadr) are converted into 15 to 270.

If the specified addresses are thus converted by setting the value ofSft to +15, the values of specified addresses obtained by the conversionSadr corresponding to input gray level data 241 to 255 become largerthan the maximum value 255 of the base addresses Oadr. Consequently,there exist no output gray level data corresponding to the input graylevel data 241 to 255.

In such a case, it is considered that the specified addresses Sadrcorresponding to the input gray level data 241 to 255 are set to themaximum value 255 of the base addresses Oadr. Consequently, output graylevel data corresponding to the input gray level data 241 to 255 are thesame as output gray level data corresponding to the input gray leveldata 240. The output gray level data corresponding to the input graylevel data 241 to 255 are graphically indicated by a straight portionb10 in FIG. 13.

In the fifth method, the output gray level data corresponding to theinput gray level data which correspond to the specified addresses Sadrwhose values are larger than 255 (the input gray levels 241 to 255 inthe above described example) are found on the basis of a predeterminedrule from total output gray level data corresponding to the input graylevel data which corresponds to the specified address Sadr whose valueis 255 (the input gray level 240 in the above described example). Thisrule is so determined that total gray levels corresponding to the inputgray level data 241 to 255 are increased one at a time in ascendingorder of input gray levels from a total gray level corresponding to theinput gray level data 240.

Such a rule is not limited to the foregoing rule. That is, other rulesmay be used. For example, the rule may be so determined that total graylevels corresponding to the input gray level data 241 to 255 areincreased a predetermined value other than one at a time in ascendingorder of input gray levels from the total gray level corresponding tothe input gray level data 240. In addition, the rule may be sodetermined that total gray levels corresponding to the input gray leveldata 241 to 255 are increased a predetermined value at a time inascending order of input gray levels and every a predetermined number ofinput gray levels from the total gray level corresponding to the inputgray level data 240.

The relationship between the input gray level data and the output graylevel data in a case where input gray level-output gray level datacorresponding to the input gray levels 241 to 255 are so corrected thatthe total gray levels corresponding to the input gray level data 241 to255 are increased one at a time in ascending order of input gray levelsfrom the total gray level corresponding to the input gray level data 240is indicated by a graph line b2 in FIG. 14. This graph line b2 isobtained by shifting the graph line a in FIG. 14 by a predeterminedamount to the left and indicating input gray level-output gray levelcharacteristics corresponding to the input gray levels 241 to 255 by astraight portion b20 which is a straight line passing through a point,which corresponds to the input gray level data 240, on a graph lineobtained by the shifting and having a gradient θ of 45°.

(V-2-2) Description is made of a processing method actually carried outin a case where the designated copy density is higher than the referencecopy density.

In the actual processing, specified addresses are not converted in theaddress generating circuit 12. Specifically, an address specifyingsignal used as the basis is outputted from the address generatingcircuit 12. The CPU 10 reads out input gray level-output gray level datacorresponding to a developing color and the designated copy density fromthe data ROM 11 and transfers the same to the RAM 14 on the basis of thedeveloping color and the designated copy density. Transfer destinationaddresses to the table memory 13 corresponding to the input graylevel-output gray level data transferred to the RAM 14 are converted onthe basis of the foregoing expression (10).

If the designated copy density is higher than the reference copy densityand the value of Sft in the foregoing expression (10) is +15, the basetransfer destination addresses OTadr 0 to 255 corresponding to therespective data are converted into new transfer destination addressesNTadr -15 to 240. Data corresponding to the new transfer destinationaddresses NTadr whose values are smaller than the minimum value 0 of thebase transfer destination addresses OTadr are deleted.

By the conversion of the transfer destination addresses, blank transferdestination addresses at which there exist no data to be transferred(addresses 241 to 255 in the above described example) are generated in aportion assigned large addresses in each of the areas TE₀, TE₁, TE₂ andTE₃ in the table memory 13. Therefore, output gray level datacorresponding to the blank transfer destination addresses are generatedin the following manner.

The output gray level data corresponding to the blank transferdestination addresses are found on the basis of a predetermined rulefrom output gray level data corresponding to the maximum value of thenew transfer destination addresses NTadr. This rule is so determinedthat total gray levels corresponding to the respective blank transferdestination addresses are increased one at a time in ascending order ofblank transfer destination addresses from the total gray levelcorresponding to the maximum value of the new transfer destinationaddresses NTadr (240 in the above described example).

If output gray level data corresponding to the whole of each of areasTE₀, TE₁, TE₂ and TE₃ in the table memory 13 are thus generated in theRAM 14, the data are transferred to the table memory 13 from the RAM 14.Thereafter, if an address specifying signal is outputted from theaddress generating circuit 12, data is outputted from a correspondingaddress in the table memory 13.

If the input gray level-output gray level characteristics obtained bythe above described fifth method which are indicated by the graph lineb2 or c2 in FIG. 12 or 14 are compared with the original input graylevel-output gray level characteristics which are indicated by the graphline a in FIG. 12 or 14, the value of an output gray level at a point onthe characteristic curve b2 or c2 is unchanged from the value of anoutput gray level at a corresponding point on the characteristic curvea. Accordingly, the relationship between the output gray level data andthe density at which printing is actually done in the original inputgray level-output gray level characteristics can be maintained.Therefore, the relationship between the input gray level data and thedensity at which printing is actually done can be made linear. Inaddition, the range of output gray levels is not extremely decreased, ascompared with that in the method of merely adding or subtracting anoffset value to or from an output gray level found by the original inputgray level-output gray level data.

(VI) Sixth Method

The sixth method is one for converting the original input graylevel-output gray level characteristics (indicated by the graph line ain FIG. 12 or 14) into characteristics obtained by shifting the graphline a2 to the left or right (indicated by the graph line b2 or c2)depending on the designated copy density and then, finding output graylevels relative to input gray levels at which there exist no output graylevel data by a method different from the fifth method.

(VI-1) Description is now made of the idea of the sixth method by takingas an example a case where the designated copy density is higher thanthe reference copy density.

When the designated copy density is higher than the reference copydensity, the original input gray level-output gray level characteristicsare converted into characteristics obtained by shifting the graph line aindicating the original input gray level-output gray levelcharacteristics to the left, as indicated by the graph line b1 in FIG.13. The amount of the shifting is set to a value corresponding to thedesignated copy density. If the graph line a indicating the originalinput gray level-output gray level characteristics is merely shifted tothe left, no input gray level-output gray level data exist in a portionat high input gray levels. Accordingly, input gray level-output graylevel data in the portion are indicated by a graph line b20 in FIG. 14which is a straight line having a gradient θ of, for example, 45°. Theinput gray levels are thus converted on the basis of the input graylevel-output gray level data b2 found.

(VI-1-1) Description is made of a case where the designated copy densityis higher than the reference copy density using the idea of convertingspecified addresses. The original input gray level-output gray leveldata in indicated by the graph line a in FIGS. 13 and 14 is shown inTable 5.

                                      TABLE 5                                     __________________________________________________________________________    INPUT                                                                         GRAY BASE    OUTPUT GRAY                                                                            TOTAL      DITHER                                       LEVEL                                                                              ADDRESS LEVEL DATA                                                                             GRAY DITHER                                                                              FATTING                                      DATA (Oadr)  G.sub.0                                                                         G.sub.1                                                                          G.sub.2                                                                         G.sub.3                                                                         LEVEL                                                                              STEP  PATTERN                                      __________________________________________________________________________    0    0       2 1  1 2 6    0     --                                           1    1       2 1  2 2 7    1     2                                            2    2       2 2  2 2 8    2     1                                            3    3       3 2  2 2 9    3     0                                            4    4       3 2  2 3 10   0     3                                            5    5       3 2  3 3 11   1     2                                            6    6       3 3  3 3 12   2     1                                            7    7       4 3  3 3 13   3     0                                            8    8       4 3  3 4 14   0     3                                            9    9       4 3  4 4 15   1     2                                            10   10      4 4  4 4 16   2     1                                            .    .       . .  . . .    .     .                                            .    .       . .  . . .    .     .                                            .    .       . .  . . .    .     .                                            248  248     55                                                                              54 54                                                                              55                                                                              218  0     3                                            249  249     55                                                                              54 55                                                                              55                                                                              219  1     2                                            250  250     55                                                                              55 55                                                                              55                                                                              220  2     1                                            251  251     56                                                                              55 55                                                                              55                                                                              221  3     0                                            252  252     56                                                                              55 55                                                                              56                                                                              222  0     3                                            253  253     56                                                                              55 56                                                                              56                                                                              223  1     2                                            254  254     56                                                                              56 56                                                                              56                                                                              224  2     1                                            255  255     57                                                                              56 56                                                                              56                                                                              225  3     0                                            __________________________________________________________________________

In Table 5, the dither step indicates the order of threshold values inthe dither matrix used for generating input gray level-output gray leveldata which corresponds to a gray scale pattern in a dither matrix of aBayer type, a spiral type, a dot type or the like. In this example,there are four dither steps 0, 1, 2 and 3. The dither steps 0, 1, 2 and3 are repeated in sets of four and are respectively assigned to inputgray level data in ascending order of their input gray levels.

Furthermore, the dither fatting pattern indicates which of output graylevels corresponding to the pixels G₀, G₁, G₂ and G₃ in the dithermatrix with respect to an input gray level is made higher than an outputgray level corresponding to the corresponding pixel with respect to aninput gray level which is lower by one. When an output gray levelcorresponding to the pixel G₀ out of the pixels G₀, G₁, G₂ and G₃ withrespect to an input gray level is higher than an output gray levelcorresponding to the pixel G₀ with respect to an input gray level whichis lower by one, the dither fatting pattern is 0. When an output graylevel corresponding to the pixel G₁ out of the pixels with respect to aninput gray level is higher than an output gray level corresponding tothe pixel G₁ with respect to an input gray level which is lower by one,the dither fatting pattern is 1. When an output gray level correspondingto the pixel G₂ out of the pixels with respect to an input gray level ishigher than an output gray level corresponding to the pixel G₂ withrespect to an input gray level which is lower by one, the dither fattingpattern is 2. When an output gray level corresponding to the pixel G₃out of the pixels with respect to an input gray level is higher than anoutput gray level corresponding to the pixel G ₃ with respect to aninput gray level which is lower by one, the dither fatting pattern is 3.

The characteristics b1 obtained by shifting the graph line a indicatingthe original input gray level-output gray level characteristics shown inFIG. 13 to the left are obtained by converting base addresses Oadr bythe same expression (11) as the foregoing expression (1):

    Sadr=Oadr+Sft                                              (11)

If the value of Sft corresponding to the characteristics b1 shown inFIG. 13 is +15, the base addresses Oadr 0 to 255 are converted intospecified addresses Sadr 15 to 270 by the expression (11). If thespecified addresses are thus converted by setting the value of Sft to+15, the values of the specified addresses obtained by the conversionSadr corresponding to the input gray level data 241 to 255 become largerthan the maximum value 255 of the base addresses Oadr. Consequently,there exist no output gray level data corresponding to the input graylevel data 241 to 255.

In such a case, it is considered that the specified addresses Sadrcorresponding to the input gray level data 241 to 255 are set to themaximum value 255 of the base addresses Oadr. Consequently, output graylevel data corresponding to the input gray level data 241 to 255 are thesame as output gray level data corresponding to the input gray leveldata 240. The specified addresses Sadr and the output gray level datacorresponding to the input gray level data in such a case are as shownin Table 6. In addition, the output gray level data corresponding to theinput gray level data 241 to 255 are graphically indicated by a straightportion b10 in FIG. 13.

                                      TABLE 6                                     __________________________________________________________________________    INPUT                                                                         GRAY SPECIFIED                                                                             OUTPUT GRAY                                                                            TOTAL      DITHER                                       LEVEL                                                                              ADDRESS LEVEL DATA                                                                             GRAY DITHER                                                                              FATTING                                      DATA (Sadr)  G.sub.0                                                                         G.sub.1                                                                          G.sub.2                                                                         G.sub.3                                                                         LEVEL                                                                              STEP  PATTERN                                      __________________________________________________________________________    0    15      6 5  5 5 21   3     0                                            1    16      6 5  5 6 22   0     3                                            2    17      6 5  6 6 23   1     2                                            3    18      6 6  6 6 24   2     1                                            4    19      7 6  6 6 25   3     0                                            .    .       . .  . . .    .     .                                            .    .       . .  . . .    .     .                                            .    .       . .  . . .    .     .                                            233  248     55                                                                              54 54                                                                              55                                                                              218  0     3                                            234  249     55                                                                              54 55                                                                              55                                                                              219  1     2                                            235  250     55                                                                              55 55                                                                              55                                                                              220  2     1                                            236  251     56                                                                              55 55                                                                              55                                                                              221  3     0                                            237  252     56                                                                              55 55                                                                              56                                                                              222  0     3                                            238  253     56                                                                              55 56                                                                              56                                                                              223  1     2                                            239  254     56                                                                              56 56                                                                              56                                                                              224  2     1                                            240  255     57                                                                              56 56                                                                              56                                                                              225  3     0                                            241  255     57                                                                              56 56                                                                              56                                                                              225  --    --                                           242  255     57                                                                              56 56                                                                              56                                                                              225  --    --                                           243  255     57                                                                              56 56                                                                              56                                                                              225  --    --                                           .    .       . .  . . .    --    --                                           .    .       . .  . . .    --    --                                           .    .       . .  . . .    --    --                                           255  255     57                                                                              56 56                                                                              56                                                                              225  --    --                                           __________________________________________________________________________

In the sixth method, the output gray level data corresponding to theinput gray level data which correspond to the specified addresses Sadrwhose values are larger than 225 (the input gray levels 241 to 255 inthe above described example) are found in the following manner.

First, dither fatting patterns corresponding to dither steps are foundon the basis of output gray level data with respect to continuous fourinput gray level data. For example, dither fatting patternscorresponding to dither steps are found on the basis of output graylevels with respect to an input gray level which corresponds to thespecified address Sadr whose value is 255 (the input gray 240 in theabove described example) and three input gray levels sequentiallydecreased one at a time from the input gray level (the input gray levels239, 238 and 237 in the above described example).

The dither steps corresponding to the input gray levels 237 to 240 arerespectively found from values left after the specified addressesobtained by the conversion Sadr corresponding to the input gray levels237 to 240 divided by 4, and are respectively 0, 1, 2 and 3. Inaddition, the dither fatting patterns corresponding to the input graylevels 237 to 240 are found by, for example, checking to see which ofoutput gray levels corresponding to the pixels G₀, G₁, G₂ and G₃ withrespect to each of the input gray levels is higher than an output graylevel corresponding to the corresponding pixel with respect to an inputgray level which is lower by one, and are respectively 3, 2, 1 and 0.That is, the dither steps 0, 1, 2 and 3 correspond to the dither fattingpatterns 3, 2, 1 and 0.

The dither steps corresponding to the input gray levels 241 to 255 whichcorrespond to the specified addresses Sadr whose values are larger than255 are then found. The dither steps corresponding to the input graylevels 241 to 255 are respectively found from values left after thespecified addresses obtained by the conversion Sadr corresponding to theinput gray levels 241 to 255 divided by 4. The dither fatting patternscorresponding to the input gray levels 241 to 255 are then found on thebasis of the relationship between the dither steps which have beenalready found and the dither fatting patterns and the dither stepscorresponding to the input gray levels 241 to 255.

Output gray levels corresponding to the input gray levels 241 to 255which correspond to the specified addresses obtained by the conversionSadr whose values are larger than 255 are found using the dither fattingpatterns found in the above described manner from the output gray levelcorresponding to the input gray level 240 which corresponds to thespecified address Sadr whose value is 255.

Table 7 shows the output gray level data corresponding to the input graylevel data 241 to 255 found in the above described manner. In addition,the relationship between the input gray level data and the output graylevel data in a case where input gray level-output gray level datacorresponding to the input gray levels 241 to 255 are thus corrected isindicated by the graph line b2 in FIG. 14. This graph line b2 isobtained by shifting the graph line a in FIG. 14 by a predeterminedamount to the left and indicating input gray level-output gray levelcharacteristics corresponding to the input gray levels 241 to 255 by astraight portion b20 which is a straight line passing through a point,which corresponds to the input gray level 240, on a graph line obtainedby the shifting and having a gradient θ of 45°.

                                      TABLE 7                                     __________________________________________________________________________    INPUT                                                                         GRAY SPECIFIED                                                                             OUTPUT GRAY                                                                            TOTAL      DITHER                                       LEVEL                                                                              ADDRESS LEVEL DATA                                                                             GRAY DITHER                                                                              FATTING                                      DATA (Sadr)  G.sub.0                                                                         G.sub.1                                                                          G.sub.2                                                                         G.sub.3                                                                         LEVEL                                                                              STEP  PATTERN                                      __________________________________________________________________________    .    .       . .  . . .    .     .                                            .    .       . .  . . .    .     .                                            .    .       . .  . . .    .     .                                            237  252     56                                                                              55 55                                                                              56                                                                              222  0     3                                            238  253     56                                                                              55 56                                                                              56                                                                              223  1     2                                            239  254     56                                                                              56 56                                                                              56                                                                              224  2     1                                            240  255     57                                                                              56 56                                                                              56                                                                              225  3     0                                            241  256     57                                                                              56 56                                                                              57                                                                              226  (0)   (3)                                          242  257     57                                                                              56 57                                                                              57                                                                              227  (1)   (2)                                          243  258     57                                                                              57 57                                                                              57                                                                              228  (2)   (1)                                          244  259     58                                                                              57 57                                                                              57                                                                              229  (3)   (0)                                          245  260     58                                                                              57 57                                                                              58                                                                              230  (0)   (3)                                          246  261     58                                                                              57 58                                                                              58                                                                              231  (1)   (2)                                          247  262     58                                                                              58 58                                                                              58                                                                              232  (2)   (1)                                          248  263     59                                                                              58 58                                                                              58                                                                              233  (3)   (0)                                          249  264     59                                                                              58 58                                                                              59                                                                              234  (0)   (3)                                          250  265     59                                                                              58 59                                                                              59                                                                              235  (1)   (2)                                          251  266     59                                                                              59 59                                                                              59                                                                              236  (2)   (1)                                          252  267     60                                                                              59 59                                                                              59                                                                              237  (3)   (0)                                          253  268     60                                                                              59 59                                                                              60                                                                              238  (0)   (3)                                          254  269     60                                                                              59 60                                                                              60                                                                              239  (1)   (2)                                          255  270     60                                                                              60 60                                                                              60                                                                              240  (2)   (1)                                          __________________________________________________________________________

(VI-1-2) Description is now made of processing actually performed.

In the actual processing, specified addresses are not converted in theaddress generating circuit 12. Specifically, an address specifyingsignal used as the basis corresponding to a pixel in the dither matrixwhich corresponds to input gray level data and a pixel position signalis outputted from the address generating circuit 12. The CPU 10 readsout input gray level-output gray level data corresponding to adeveloping color and the designated copy density from the data ROM 11and transfers the same to the RAM 14 on the basis of the developingcolor and the designated copy density.

Transfer destination addresses to the table memory 13 corresponding tothe input gray level-output gray level data transferred to the RAM 14are converted. This conversion of the transfer destination addresses isrepresented by the same expression (12) as the foregoing expression (2):

    NTadr=OTadr-Sft                                            (12)

The value of Sft corresponding to the designated copy density ispreviously determined and stored in the data ROM 11 or the other ROM.Base transfer destination addresses OTadr corresponding to respectiveinput gray level-output gray level data to be transferred are convertedinto new transfer destination addresses NTadr on the basis of theforegoing expression (12) from the value of Sft corresponding to thedesignated copy density.

If the designated copy density is higher than the reference copy densityand the value of Sft is +15, the base transfer destination addressesOadr 0 to 255 corresponding to the respective data are converted intonew transfer destination addresses NTadr -15 to 240 by the foregoingexpression (12). Data corresponding to the new transfer destinationaddresses NTadr whose values are smaller than the minimum value 0 of thebase transfer destination addresses OTadr are deleted.

By the conversion of the transfer destination addresses, blank transferdestination addresses at which there exist no data to be transferred(addresses 241 to 255 in the above described example) are generated in aportion assigned large addresses in each of the areas TE₀, TE₁, TE₂ andTE₃ in the table memory 13. Therefore, output gray level datacorresponding to the blank transfer destination addresses are found inthe same method as the sixth method of finding the output gray leveldata corresponding to the input gray level data 241 to 255 in Table 7described in the item (VI-I-1). Therefore, output gray level datacorresponding to the blank transfer destination addresses 241 to 255(output gray level data corresponding to the input gray level data 241to 255) are the same as the output gray level data corresponding to theinput gray level data 241 to 255 in Table 7.

If output gray level data corresponding to the whole of each of theareas TE₀, TE₁, TE₂ and TE₃ in the table memory 13 are thus generated inthe RAM 14, the data are transferred to the table memory 13 from the RAM14. Thereafter, if an address specifying signal is outputted from theaddress generating circuit 12, data is outputted from a correspondingaddress in the table memory 13.

(VI-2) Description is made of a case where the designated copy densityis lower than the reference copy density.

If the designated copy density is lower than the reference copy density,the original input gray level-output gray level characteristics areconverted into characteristics obtained by shifting the graph line aindicating the original input gray level-output gray levelcharacteristics to the right, as indicated by the graph line c1 in FIG.11. The amount of the shifting is set to a value corresponding to thedesignated copy density. If the graph line a indicating the originalinput gray level-output gray level characteristics is merely shifted tothe right, no input gray level-output gray level data exist in a portionat low input gray levels. Accordingly, input gray level-output graylevel data in the portion are indicated by a graph line c20 in FIG. 12which is a straight line having a gradient θ of, for example, 45°. Theinput gray levels are thus converted on the basis of the input graylevel-output gray level data c2 found.

(VI-2-1) Description is made of a case where the designated copy densityis lower than the reference copy density using the idea of convertingspecified addresses.

If the designated copy density is lower than the reference copy densityand the value of Sft in the foregoing expression (11) is -25, specifiedaddresses corresponding to base addresses Oadr 0 to 255 are convertedinto -25 to 230. If the specified addresses are thus converted bysetting the value of Sft to -25, the values of specified addressesobtained by the conversion Sadr corresponding to the input gray leveldata 0 to 24 become smaller than the minimum value 0 of the baseaddresses Oadr. Consequently, there exist no output gray level datacorresponding to the input gray level data 0 to 24.

In such a case, it is considered that the specified addresses Sadrcorresponding to the input gray level data 0 to 24 are set to theminimum value 0 of the base addresses Oadr. Consequently, output graylevel data corresponding to the input gray level data 0 to 24 are thesame as output gray level data corresponding to the input gray leveldata 25. The output gray level data corresponding to the input graylevel data 0 to 24 in such a case are graphically indicated by astraight portion c10 in FIG. 11.

In the sixth method, the output gray level data corresponding to theinput gray level data which correspond to the specified addressesobtained by the conversion Sadr whose values are smaller than 0 (theinput gray levels 0 to 24 in the above described example) are found inthe following manner.

First, dither fatting patterns corresponding to dither steps are foundon the basis of output gray levels corresponding to continuous fourinput gray levels. Dither steps corresponding to the input gray levelswhich correspond to the specified addresses obtained by the conversionSadr whose values are smaller than 0 (the input gray levels 0 to 24 inthe above described example) are then found, to find dither fattingpatterns corresponding to the input gray levels 0 to 24.

The output gray levels corresponding to the input gray levels 0 to 24which correspond to the specified addresses Sadr whose values aresmaller than 0 are found using the dither fatting patterns found in theabove described manner from the output gray level corresponding to theinput gray level which corresponds to the specified address Sadr whosevalue is 0 (the input gray level 25 in the above described example).

The relationship between the input gray level data and the output graylevel data in a case where input gray level-output gray level datacorresponding to the input gray levels 0 to 24 thus found are correctedis indicated by the graph line c2 in FIG. 12. This graph line c2 isobtained by shifting the graph line a in FIG. 12 by a predeterminedamount to the right and indicating input gray level-output gray levelcharacteristics corresponding to the input gray levels 0 to 24 by astraight portion c20 which is a straight line passing through a point,which corresponds to the input gray level data 25, on a graph lineobtained by the shifting and having a gradient θ of 45°.

(VI-2-2) Description is made of a processing method actually carried outin a case where the designated copy density is lower than the referencecopy density.

In the actual processing, specified addresses are not converted in theaddress generating circuit 12. Specifically, an address specifyingsignal used as the basis corresponding to a pixel in the dither matrixwhich corresponds to input gray level data and a pixel position signalis outputted from the address generating circuit 12. The CPU 10 readsout input gray level-output gray level data corresponding to adeveloping color and the designated copy density from the data ROM 11and transfers the same to the RAM 14 on the basis of the developingcolor and the designated copy density.

Transfer destination addresses to the table memory 13 corresponding tothe input gray level-output gray level data transferred to the RAM 14are converted using the foregoing expression (12) .

If the designated copy density is lower than the reference copy densityand the value of Sft in the foregoing expression (12) is -25. the basetransfer destination addresses OTadr 0 to 255 corresponding to therespective data are converted into new transfer destination addressesNTadr 25 to 280. Data corresponding to the new transfer destinationaddresses NTadr whose values are larger than the maximum value 255 ofthe base transfer destination addresses OTadr are deleted.

By the conversion of the transfer destination addresses, blank transferdestination addresses at which there exist no data to be transferred(addresses 0 to 24 in the above described example) are generated in aportion assigned small addresses in each of the areas TE₀, TE₁, TE₂ andTE₃ in the table memory 13. Therefore, output gray level datacorresponding to the blank transfer destination addresses are generatedin the same method as the sixth method of finding the output gray leveldata corresponding to the input gray level data 0 to 24 described in theitem (VI-2-1).

If output gray level data corresponding to the whole of each of theareas TE₀, TE₁, TE₂ and TE₃ in the table memory 13 are thus generated inthe RAM 14, the data are transferred to the table memory 13 from the RAM14. Thereafter, if an address specifying signal is outputted from theaddress generating circuit 12, data is outputted from a correspondingaddress in the table memory 13.

If the input gray level-output gray level characteristics obtained bythe above described sixth method which are indicated by the graph lineb2 or c2 in FIG. 12 or 14 are compared with the original input graylevel-output gray level characteristics which are indicated by the graphline a in FIG. 12 or 14, the value of an output gray level at a point onthe characteristic curve b2 or c2 is unchanged from the value of anoutput gray level at a corresponding point on the characteristic curvea. Accordingly, the relationship between the input gray level data andthe density at which printing is actually done in the original inputgray level-output gray level characteristics can be maintained.Therefore, the relationship between the input gray level data and thedensity at which printing is actually done can be made linear. Inaddition, the range of output gray levels is not extremely decreased, ascompared with that in the method of merely adding or subtracting anoffset value to or from an output gray level found by the original inputgray level-output gray level data.

(VII) Seventh Method

As shown in FIG. 16, the seventh method is one for rotating, in a caseinput gray levels are used to enter the axis of abscissa, output graylevels are used to enter the axis of ordinate, and the original inputgray level-output gray level characteristics are indicated by a graphline a, the graph line a through a predetermined angle in a clockwisedirection or in a counterclockwise direction around a point, whichcorresponds to the highest output gray level, on the graph line adepending on the designated copy density, to convert the original inputgray level-output gray level characteristics into characteristicscorresponding to the designated copy density, and finding output graylevels relative to input gray levels at which there exist no output graylevel data as the result of the above described conversion by apredetermined operation.

(VII-1) Description is now made of the idea of the seventh method bytaking as an example a case where the designated copy density is lowerthan the reference copy density.

FIGS. 15 and 16 show input gray level-output gray level characteristicsin a case where input gray levels are used to enter the axis of abscissaand output gray levels are used to enter the axis of ordinate. A graphline a shown in FIGS. 15 and 16 indicates the original input graylevel-output gray level characteristics. When the designated copydensity is lower than the reference copy density, the original inputgray level-output gray level characteristics are converted intocharacteristics obtained by rotating the graph line a indicating theoriginal input gray level-output gray level characteristics through apredetermined angle in a counterclockwise direction around a point,which corresponds to the highest output gray level, on the graph line a,as indicated by a graph line cl in FIG. 15. The angle of rotation is setto a value corresponding to the designated copy density. If the graphline a indicating the original input gray level-output gray levelcharacteristics is merely rotated in a counterclockwise direction, noinput gray level-output gray level data exist in a portion at low inputgray levels. Accordingly, input gray level-output gray level data in theportion are indicated by a graph line c20 in FIG. 16 which is a straightline having a gradient θ of, for example, 45°. The input gray levels arethus converted on the basis of input gray level-output gray level datac2 found.

(VII-1-1) Description is made of a case where the designated copydensity is lower than the reference copy density using the idea ofconverting specified addresses.

The characteristics c1 obtained by rotating the graph line a indicatingthe original input gray level-output gray level characteristics shown inFIG. 15 through a predetermined angle in a counterclockwise directionare obtained by converting base addresses Oadr by the same expression(13) as the foregoing expression (3):

    Sadr=255-{Gain (255-Oadr)+Offset}                          (13)

In this case, the value of Offset is always 0. Gain is so determined, ifthe designated copy density is higher than the reference copy density,that it takes a value smaller than 1 and that the higher the designatedcopy density is, the smaller the value is. On the other hand, Gain is sodetermined, if the designated copy density is lower than the referencecopy density, that it takes a value larger than 1 and that the lower thedesignated copy density is, the larger the value is.

If the specified addresses are converted by setting the value of Gaincorresponding to the characteristics c1 shown in FIG. 15 to 1.1, thevalues of specified addresses obtained by the conversion Sadrcorresponding to input gray level data 0 to (xL-1) (see FIGS. 15 and 16)become smaller than the minimum value 0 of the base addresses Oadr.Consequently, there exist no output gray level data corresponding to theinput gray level data 0 to (xL-1).

In such a case, it is considered that the specified addresses Sadrcorresponding to the input gray level data 0 to (xL-1) are set to theminimum value 0 of the base addresses Oadr. Consequently, output graylevel data corresponding to the input gray level data 0 to (xL-1) arethe same as output gray level data corresponding to the input gray leveldata xL. The output gray level data corresponding to the input graylevel data 0 to (xL-1) are graphically indicated by a straight portionc10 in FIG. 15.

In the seventh method, the output gray level data corresponding to theinput gray level data which correspond to the specified addresses Sadrwhose values are smaller than 0 (the input gray levels 0 to (xL-1) inthe above described example) are found in the same method as the fifthmethod described in the item (V-1-1) or the sixth method described inthe item (VI-2-1). Input gray level-output gray level characteristicsthus obtained are indicated by the graph line c2 in FIG. 16.Specifically, this graph line c2 is obtained by rotating the graph linea indicating the original input gray level-output gray levelcharacteristics through a predetermined angle in a counterclockwisedirection around a point, which corresponds to the highest output graylevel, on the graph line a and indicating input gray level-output graylevel characteristics corresponding to the input gray levels 0 to (xL-1)by a straight portion c20 which is a straight line passing through apoint, which corresponds to the input gray level data xL, on a graphline obtained by the rotation and having a gradient θ of 45°.

(VII-1-2) Description is now made of processing actually performed.

In the actual processing, specified addresses are not converted in theaddress generating circuit 12. Specifically, an address specifyingsignal used as the basis corresponding to a pixel in the dither matrixwhich corresponds to input gray level data and a pixel position signalis outputted from the address generating circuit 12. The CPU 10 readsout input gray level-output gray level data corresponding to adeveloping color and the designated copy density from the data ROM 11and transfers the same to the RAM 14 on the basis of the developingcolor and the designated copy density.

This conversion of the transfer destination addresses is represented bythe same expression (14) as the foregoing expression (4):

    NTadr={OTadr+255 (Gain-1)+Offset}÷Gain                 (14)

In this case, the value of Offset is 0. The value of Gain correspondingto the designated copy density is previously determined and stored inthe data ROM 11 or the other ROM. Base transfer destination addressesOTadr corresponding to respective input gray level-output gray leveldata in each of the areas TE₀, TE₁, TE₂ and TE₃ corresponding to thepixels G₀, G₁, G₂ and G₃ in the dither matrix are converted into newtransfer destination addresses NTadr on the basis of the foregoingexpression (14) from the value of Gain corresponding to the designatedcopy density.

If the designated copy density is lower than the reference copy density(Gain<1), blank transfer destination addresses at which there exist nodata to be transferred are generated in a portion assigned smalladdresses in each of the areas TE₀, TE₁, TE₂ and TE₃ in the table memory13 by the conversion of the transfer destination addresses. Therefore,output gray level data corresponding to the blank transfer destinationaddresses are generated.

If the blank transfer destination addresses at which there exist no datato be transferred are generated in a portion assigned small addresses ineach of the areas TE₀, TE₁, TE₂ and TE₃ in the table memory 13, outputgray level data corresponding to the blank transfer destinationaddresses are found in the same method as the fifth method described inthe item (V-1-2) or the sixth method described in the item (VI-2-2). Ifoutput gray level data corresponding to the whole of each of the areasTE₀, TE₁, TE₂ and TE₃ in the table memory 13 are thus generated in theRAM 14, the data are transferred to the table memory 13 from the RAM 14.Thereafter, if an address specifying signal is outputted from theaddress generating circuit 12, data is outputted from a correspondingaddress in the table memory 13.

(VII-2) Description is made of a case where the designated copy densityis higher than the reference copy density.

If the designated copy density is higher than the reference copydensity, the original input gray level-output gray level characteristicsare converted into characteristics obtained by rotating the graph line aindicating the original input gray level-output gray levelcharacteristics shown in FIG. 15 or 16 through a predetermined angle ina clockwise direction around a point, which corresponds to the highestoutput gray level, on the graph line a. The input gray levels areconverted on the basis of input gray level-output gray level dataobtained by the conversion.

If the designated copy density is higher than the reference copy density(Gain<1), transfer destination addresses are converted by the foregoingexpression (14), so that the original input gray level-output gray leveldata is transferred to the table memory 13. In this case, the values ofnew transfer destination addresses NTadr may, in some cases, be smallerthan the minimum value 0 of the base addresses Oadr. However, datacorresponding to the new transfer destination addresses are nottransferred.

If the input gray level-output gray level characteristics obtained bythe above described seventh method are compared with the original inputgray level-output gray level characteristics, the value of an outputgray level at a point on the characteristic curve is slightly changedfrom the value of an output gray level at a corresponding point on theoriginal characteristic curve in a low density portion. However, therange of input gray levels which can be reproduced is not decreased, ascompared with that in the fifth method.

(VIII) Eighth Method

As shown in FIG. 18, the eighth method is one for rotating, in a casewhere input gray levels are used to enter the axis of abscissa, outputgray levels are used to enter the axis of ordinate, and the originalinput gray level-output gray level characteristics are indicated by agraph line a, the graph line a through a predetermined angle in aclockwise direction or in a counterclockwise direction around a point,which corresponds to the lowest output gray level, on the graph line adepending on the designated copy density, to convert the input graylevel-output gray level characteristics into characteristicscorresponding to the designated copy density, and finding output graylevels relative to input gray levels at which there exists no outputgray level data as the result of the above described conversion by apredetermined operation.

(VIII-1) Description is now made of the idea of the eighth method bytaking as an example a case where the designated copy density is higherthan the reference copy density.

FIGS. 17 and 18 show input gray level-output gray level characteristicsin a case where input gray levels are used to enter the axis of abscissaand output gray levels are used to enter the axis of ordinate. A graphline a shown in FIGS. 17 and 18 indicates the original input graylevel-output gray level characteristics. When the designated copydensity is higher than the reference copy density, the original inputgray level-output gray level characteristics are converted intocharacteristics obtained by rotating the graph line a indicating theoriginal input gray level-output gray level characteristics through apredetermined angle in a counterclockwise direction around a point,which corresponds to the lowest output gray level, on the graph line a,as indicated by a graph line b1 in FIG. 17. The angle of rotation is setto a value corresponding to the designated copy density. If the graphline a indicating the original input gray level-output gray levelcharacteristics is merely rotated in a counterclockwise direction, noinput gray level-output gray level data exist in a portion at high inputgray levels. Accordingly, input gray level-output gray level data in theportion are indicated by a graph line c20 in FIG. 18 which is a straightline having a gradient θ of, for example, 45°. The input gray levels arethus converted on the basis of input gray level-output copy density graylevel data c2 found.

(VIII-1-1) Description is made of a case where the designated copydensity is higher than the reference copy density using the idea ofconverting specified addresses.

The characteristics b1 obtained by rotating the graph line a indicatingthe original input gray level-output gray level characteristics shown inFIG. 17 through a predetermined angle in a counterclockwise directionare obtained by converting base addresses Oadr corresponding to inputgray level data by the same expression (15) as the foregoing expression(5):

    Sadr=255-{Gain (255-Oadr)+Offset}                          (15)

Gain is so determined, if the designated copy density is higher than thereference copy density, that it takes a value larger than 1 and that thehigher the designated copy density is, the larger the value is. On theother hand, Gain is so determined, if the designated copy density islower than the reference copy density, that it takes a value smallerthan 1 and that the lower the designated copy the density is, thesmaller the value is. In this case, the value of Offset is so adjustedthat the value of {255. Gain +Offset} becomes 255.

If the specified addresses are converted using the values of Gain (>1)and Offset corresponding to the characteristics b1 shown in FIG. 17, thevalues of specified addresses obtained by the conversion Sadrcorresponding to input gray level data (xH+1) to 255 (see FIGS. 17 and18) become larger than the maximum value 255 of the base addresses Oadr.Consequently, there exist no output gray level data corresponding to theinput gray level data (xH+1) to 255.

In such a case, it is considered that the specified address Sadrcorresponding to the input gray level data (xH +1) to set are set to themaximum value 255 of the base addresses Oadr. Consequently, output graylevel data corresponding to the input gray level data (xH+1) to 255 arethe same as output gray level data corresponding to the input gray leveldata xH. The output gray level data corresponding to the input graylevel data (xH+1) to 255 are graphically indicated by a straight portionb10 in FIG. 17.

In the eighth method, the output gray level data corresponding to theinput gray level data which correspond to the specified address Sadrwhose values are larger than 255 (the input gray levels (xH+1) to 255 inthe above described example) are found in the same method as the fifthmethod described in the item (V-2-1) or the sixth method described inthe item (VI-1-1). Input gray level-output gray level characteristicsthus obtained are indicated by a graph line b2 in FIG. 18. Specifically,this graph line b2 is obtained by rotating the graph line a indicatingthe original input gray level-output gray level characteristics througha predetermined angle in a counterclockwise direction around a point,which corresponds to the lowest output gray level, on the graph line aand indicating input gray level-output gray level characteristicscorresponding to the input gray levels (xH+1) to 255 by a straightportion b20 which is a straight line passing through a point, whichcorresponds to the input gray level data xH, on a graph line obtained bythe rotation and having a gradient θ of 45°.

(VIII-1-2) Description is now made of processing actually performed.

In the actual processing, specified addresses are not converted in theaddress generating circuit 12. Specifically, an address specifyingsignal used as the basis corresponding to a pixel in the dither matrixwhich corresponds to input gray level data and a pixel position signalis outputted from the address generating circuit 12. The CPU 10 readsout input gray level-output gray level data corresponding to adeveloping color and the designated copy density from the data ROM 11and transfers the same to the RAM 14 on the basis of the developingcolor and the designated copy density. This conversion of the transferdestination addresses is represented by the same expression (16) as theforegoing expression (6):

    NTadr={OTadr+255 (Gain-1)+Offset}÷Gain                 (16)

The values of Gain and Offset corresponding to the designated copydensity are previously determined and stored in the data ROM 11 or theother ROM. Base transfer destination addresses OTadr corresponding torespective input gray level-output gray level data in each of the areasTE₀, TE₁, TE₂ and TE₃ corresponding to the pixels G₀, G₁, G₂ and G₃ inthe dither matrix are converted into new transfer destination addressesNTadr on the basis of the foregoing expression (16) from the values ofGain and Offset corresponding to the designated copy density.

If the designated copy density is higher than the reference copy density(Gain>1), blank transfer destination addresses at which there exist nodata to be transferred are generated in a portion assigned largeaddresses in each of the areas TE₀, TE₁, TE₂ and TE₃ in the table memory13 by the conversion of the transfer destination addresses. Therefore,output gray level data corresponding to the blank transfer destinationaddresses are generated.

if the blank transfer destination addresses at which there exist no datato be transferred are generated in a portion assigned large addresses ineach of the areas TE₀, TE₁, TE₂ and TE₃ in the table memory 13, outputgray level data corresponding to the blank transfer destinationaddresses are found in the same method as the fifth method described inthe item (V-2-2) or the sixth method described in the item (VI-1-2). Ifoutput gray level data corresponding to the whole of each of the areasTE₀, TE₁, TE₂ and TE₃ in the table memory 13 are thus generated in theRAM 14, the data are transferred to the table memory 13 from the RAM 14.Thereafter, if an address specifying signal is outputted from theaddress generating circuit 12, data is outputted from a correspondingaddress in the table memory 13.

(VIII-2) Description is made of a case where the designated copy densityis higher than the reference copy density.

If the designated copy density is lower than the reference copy density,the original input gray level-output gray level characteristics areconverted into characteristics obtained by rotating the graph line aindicating the original input gray level-output gray levelcharacteristics shown in FIG. 17 or 18 through a predetermined angle ina clockwise direction around a point, which corresponds to the lowestoutput gray level, on the graph line a. The input gray levels areconverted on the basis of input gray level-output gray level dataobtained by the conversion.

If the designated copy density is lower than the reference copy density(Gain<1), transfer destination addresses are converted by the foregoingexpression (16), so that the original input gray level-output gray leveldata is transferred to the table memory 13. In this case, the values ofthe new transfer destination addresses NTadr may, in some cases, belarger than the maximum value 255 of the base addresses Oadr. However,data corresponding to the new transfer destination addresses NTadr arenot transferred.

If the input gray level-output gray level characteristics obtained bythe above described eighth method are compared with the original inputgray level-output gray level characteristics, the value of an outputgray level at a point on the characteristic curve is slightly changedfrom the value of an output gray level at a corresponding point on theoriginal characteristic curve in a low density portion. However, therange of input gray levels which can be reproduced is not decreased, ascompared with that in the first method.

(IX) Ninth Method

The ninth method is a method using the eighth method when the copydensity designated in the operating section is higher than the referencecopy density and using the seventh method when the copy densitydesignated in the operating section is lower than the reference copydensity.

Specifically, when the designated copy density is higher than thereference copy density, the eighth method is so used, in a case wherethe original input gray level-output gray level characteristics areindicated by the graph line a in FIG. 18, that the original input graylevel-output gray level characteristics are converted intocharacteristics indicated by the graph line b2. On the other hand, whenthe designated copy density is lower than the reference copy density,the seventh method is so used, in a case where the original input graylevel-output gray level characteristics are indicated by the graph linea in FIG. 16, that the original input gray level-output gray levelcharacteristics are converted into characteristics indicated by thegraph line c2. In this case, as a formula for converting transferdestination addresses, the expression (16) common to the seventh andeighth methods is used.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. In a density processing method in which inputgray level-output gray level data representing an output gray levelrelative to an input gray level are previously generated and stored in afirst storing means, the input gray level-output gray level data aretransferred to a second storing means, and an address in the secondstoring means is specified on the basis of input gray level datainputted so that output gray level data stored in the specified addressin the second storing means is outputted, wherein access addresses tothe second storing means are converted by adding or subtracting a valuecorresponding to a designated copy density, thereby to convert inputgray level-output gray level characteristics into characteristicscorresponding to the designated copy density.
 2. The density processingmethod according to claim 1, wherein the access addresses to said secondstoring means are transfer destination addresses from the first storingmeans to the second storing means corresponding to the input graylevel-output gray level data.
 3. The density processing method accordingto claim 1, wherein the access addresses to said second storing meansare specified addresses in the second storing means corresponding to theinput gray level data inputted.
 4. In a density processing method inwhich input gray level-output gray level data representing an outputgray level relative to an input gray level are previously generated andstored in a first storing means, the input gray level-output gray leveldata are transferred to a second storing means, and an address in thesecond storing means is specified on the basis of input gray level datainputted so that output gray level data stored in the specified addressin the second storing means is outputted,wherein access addresses to thesecond storing means are converted depending on a designated copydensity, thereby to convert input gray level-output gray levelcharacteristics into characteristics corresponding to the designatedcopy density, wherein the access addresses to the second storing meansare converted such that, if the designated copy density is higher than areference copy density, a rate of increase of an output gray levelrelative to an input gray level is higher in a lower density portion,and the higher the designated copy density is, the higher the rate ofincrease thereof is, thereby to convert the input gray level-output graylevel characteristics into characteristics corresponding to thedesignated copy density.
 5. The density processing method according toclaim 4, wherein the access addresses to said second storing means aretransfer destination addresses from the first storing means to thesecond storing means corresponding to the input gray level-output graylevel data.
 6. The density processing method according to claim 4,wherein the access addresses to said second storing means are specifiedaddresses in the second storing means corresponding to the input graylevel data inputted.
 7. In a density processing method in which inputgray level-output gray level data representing an output gray levelrelative to an input gray level are previously generated and stored in afirst storing means, the input gray level-output gray level data aretransferred to a second storing means, and an address in the secondstoring means is specified on the basis of input gray level datainputted so that output gray level data stored in the specified addressin the second storing means is outputted,wherein access addresses to thesecond storing means are converted depending on a designated copydensity, thereby to convert input gray level-output gray levelcharacteristics into characteristics corresponding to the designatedcopy density, wherein the access addresses to the second storing meansare converted such that, if the designated copy density is lower than areference copy density, a rate of decrease of an output gray levelrelative to an input gray level is higher in a higher density portion,and the lower the designated copy density is, the higher the rate ofdecrease thereof is, thereby to convert the input gray level-output graylevel characteristics into characteristics corresponding to thedesignated copy density.
 8. The density processing method according toclaim 7, wherein the access addresses to said second storing means aretransfer destination addresses from the first storing means to thesecond storing means corresponding to the input gray level-output graylevel data.
 9. The density processing method according to claim 7,wherein the access addresses to said second storing means are specifiedaddresses in the second storing means corresponding to the input graylevel data inputted.
 10. In a density processing method in which inputgray level-output gray level data representing an output gray levelrelative to an input gray level are previously generated and stored in afirst storing means, the input gray level-output gray level data aretransferred to a second storing means, and an address in the secondstoring means is specified on the basis of input gray level datainputted so that output gray level data stored in the specified addressin the second storing means is outputted,wherein access addresses to thesecond storing means are converted depending on a designated copydensity, thereby to convert input gray level-output gray levelcharacteristics into characteristics corresponding to the designatedcopy density, wherein the access addresses to the second storing meansare converted such that, if the designated copy density is higher than areference copy density, a rate of increase of an output gray levelrelative to an input gray level is higher in a lower density portion,and the higher the designated copy density is, the higher the rate ofincrease thereof is, and wherein the access addresses to the secondstoring means are converted such that, if the designated copy density islower than the reference copy density, a rate of decrease of an outputgray level relative to an input gray level is higher in a higher densityportion, and the lower the designated copy density is, the higher therate of decrease thereof is, thereby to convert the input graylevel-output gray level characteristics into characteristicscorresponding to the designated copy density.
 11. The density processingmethod according to claim 10, wherein the access addresses to saidsecond storing means are transfer destination addresses from the firststoring means to the second storing means corresponding to the inputgray level-output gray level data.
 12. The density processing methodaccording to claim 10, wherein the access addresses to said secondstoring means are specified addresses in the second storing meanscorresponding to the input gray level data inputted.
 13. In a densityprocessing method in which input gray level-output gray level data arepreviously generated and stored in a first storing means, the input graylevel-output gray level data are transferred to a second storing means,and an address in the second storing means is specified on the basis ofinput gray level data inputted so that output gray level data stored inthe specified address in the second storing means is outputted,thedensity processing method comprising the steps of: converting transferdestination addresses to the second storing means corresponding to theinput gray level-output gray level data depending on a designated copydensity; finding, when blank transfer destination addresses which do notexist in transfer destination addresses obtained by the conversion existin portions assigned small addresses of storage areas holding the inputgray level-output gray level data in the second storing means, outputgray level data corresponding to the blank transfer destinationaddresses on the basis of output gray level data corresponding to aminimum value of the transfer destination addresses obtained by theconversion; and generating new output gray level data corresponding toall the storage areas holding the input gray level-output gray leveldata in the second storing means from said steps to transfer the same tothe second storing means.
 14. In a density processing method in whichinput gray level-output gray level data are previously generated andstored in a first storing means, the input gray level-output gray leveldata are transferred to a second storing means, and an address in thesecond storing means is specified on the basis of input gray level datainputted so that output gray level data stored in the specified addressin the second storing means is outputted,the density processing methodcomprising the steps of: converting transfer destination addresses tothe second storing means corresponding to the input gray level-outputgray level data depending on a designated copy density; finding, whenblank transfer destination addresses which do not exist in transferdestination addresses obtained by the conversion exist in portionsassigned large addresses in storage areas holding the input gray leveloutput gray level data in the second storing means, output gray leveldata corresponding to the blank transfer destination addresses on thebasis of output gray level data corresponding to a maximum value of thetransfer destination addresses obtained by the conversion; andgenerating new output gray level data corresponding to all the storageareas holding the input gray level-output gray level data in the secondstoring means from said steps to transfer the same to the second storingmeans.
 15. In a density processing method in which input graylevel-output gray level data are previously generated and stored in afirst storing means, the input gray level-output gray level data aretransferred to a second storing means, and an address in the secondstoring means is specified on the basis of input gray level datainputted so that output gray level data stored in the specified addressin the second storing means is outputted,the density processing methodcomprising the steps of: converting transfer destination addresses tothe second storing means corresponding to the input gray level-outputgray level data depending on a designated copy density; finding, whenblank transfer destination addresses which do not exist in transferdestination addresses obtained by the conversion exist in portionsassigned small addresses in storage areas holding the input gray leveloutput gray level data in the second storing means, output gray leveldata corresponding to the blank transfer destination addresses on thebasis of output gray level data corresponding to a minimum value of thetransfer destination addresses obtained by the conversion; finding, whenblank transfer destination addresses which do not exist in the transferdestination addresses obtained by the conversion exist in portionsassigned large addresses in the storage areas holding the input graylevel-output gray level data in the second storing means, output graylevel data corresponding to the blank transfer destination addresses onthe basis of output gray level data corresponding to a maximum value ofthe transfer destination addresses obtained by the conversion; andgenerating new output gray level data corresponding to all the storageareas holding the input gray level-output gray level data in the secondstoring means from said steps to transfer the same to the second storingmeans.
 16. In a density processing method in which input graylevel-output gray level data are previously generated using a dithermatrix and stored in a first storing means, the input gray level-outputgray level data are transferred to a second storing means, and anaddress in the second storing means is specified on the basis of inputgray level data inputted so that output gray level data stored in thespecified address in the second storing means is outputted,the densityprocessing method comprising the steps of: converting transferdestination addresses to the second storing means corresponding to theinput gray level-output gray level data depending on a designated copydensity; finding, when blank transfer destination addresses which do notexist in transfer destination addresses obtained by the conversion existin portions assigned small addresses in storage areas holding the inputgray level-output gray level data in the second storing means, patternsof output gray level data corresponding to the blank transferdestination addresses on the basis of the original input graylevel-output gray level data to find the output gray level datacorresponding to the blank transfer destination addresses on the basisof the patterns found and output gray level data corresponding to aminimum value of the transfer destination addresses obtained by theconversion; and generating new output gray level data corresponding toall the storage areas holding the input gray level-output gray leveldata in the second storing means from said steps to transfer the same tothe second storing means.
 17. In a density processing method in whichinput gray level-output gray level data are previously generated using adither matrix and stored in a first storing means, the input graylevel-output gray level data are transferred to a second storing means,and an address in the second storing means is specified on the basis ofinput gray level data inputted so that output gray level data stored inthe specified address in the second storing means is outputted,thedensity processing method comprising the steps of: converting transferdestination addresses to the second storing means corresponding to theinput gray level-output gray level data depending on a designated copydensity; finding, when blank transfer destination addresses which do notexist in transfer destination addresses obtained by the conversion existin portions assigned large addresses in storage areas holding the inputgray level output gray level data in the second storing means, patternsof output gray level data corresponding to the blank transferdestination addresses on the basis of the original input graylevel-output gray level data to find the output gray level datacorresponding to the blank transfer destination addresses on the basisof the patterns found and output gray level data corresponding to amaximum value of the transfer destination addresses obtained by theconversion; and generating new output gray level data corresponding toall the storage areas holding the input gray level-output gray leveldata in the second storing means from said steps to transfer the same tothe second storing means.
 18. In a density processing method in whichinput gray level-output gray level data are previously generated using adither matrix and stored in a first storing means, the input graylevel-output gray level data are transferred to a second storing means,and an address in the second storing means is specified on the basis ofinput gray level data inputted so that output gray level data stored inthe specified address in the second storing means is outputted,thedensity processing method comprising the steps of: converting transferdestination addresses to the second storing means corresponding to theinput gray level-output gray level data depending on a designated copydensity; finding, when blank transfer destination addresses which do notexist in transfer destination addresses obtained by the conversion existin portions assigned small addresses in storage areas holding the inputgray level output gray level data in the second storing means, patternsof output gray level data corresponding to the blank transferdestination addresses on the basis of the original input graylevel-output gray level data to find the output gray level datacorresponding to the blank transfer destination addresses on the basisof the patterns found and output gray level data corresponding to aminimum value of the transfer destination addresses obtained by theconversion; finding, when blank transfer destination addresses which donot exist in the transfer destination addresses obtained by theconversion exist in portions assigned large addresses in the storageareas holding the input gray level-output gray level data in the secondstoring means, patterns of output gray level data corresponding to theblank transfer destination addresses on the basis of the original inputgray level-output gray level data to find the output gray level datacorresponding to the blank transfer destination addresses on the basisof the patterns found and output gray level data corresponding to amaximum value of the transfer destination addresses obtained by theconversion; and generating new output gray level data corresponding toall the storage areas holding the input gray level-output gray leveldata in the second storing means from said steps to transfer the same tothe second storing means.